Two years into his apprenticeship Booth was converted to 'salvation' and Methodism. He then read extensively and trained himself in writing and in speech, becoming a Methodist lay preacher. Booth was encouraged to be an evangelist primarily through his best friend, Will Sansom. Sansom and Booth both began in the 1840s to preach to the poor and the "sinners" of Nottingham, and Booth would probably have remained as Sansom's partner in his new "Mission" ministry, as Sansom titled it, had Sansom not died of tuberculosis, in 1848.

When his apprenticeship ended in 1848, Booth spent a year looking in vain for more suitable work than pawnbroking, which he disliked and considered ungodly. In 1849, Booth reluctantly left his family and moved to London, where he found work and lodging in a pawnbroker's shop. Booth tried to continue lay preaching in London, but the small amount of preaching work that came his way frustrated him, and so he resigned as a lay preacher and took to open-air evangelising in the streets and on Kennington Common.

In 1851, Booth joined the 'Reformers' (Methodist Reform Church), and on 10 April 1852, his 23rd birthday, he left pawnbroking and became a full-time preacher at their headquarters at Binfield Chapel in Clapham. William styled his preaching after the revivalist American James Caughey, who had made frequent visits to England and preached at Booth's favourite church, Broad Street Chapel. Just over a month after he started full-time preaching, on 15 May 1852, William Booth became formally engaged to Catherine Mumford. In November 1853, Booth was invited to become the Reformers' minister at Spalding, in Lincolnshire. Booth married Catherine Mumford on 16 June 1855 at Stockwell Green Congregational Church in London. Their wedding was very simple, as they wanted to use their time and money for his ministry. Even on their honeymoon Booth was asked to speak at meetings.

Though Booth became a prominent Methodist evangelist, he was unhappy that the annual conference of the denomination kept assigning him to a pastorate, the duties of which he had to neglect to respond to the frequent requests that he do evangelistic campaigns. At the Liverpool conference in 1861, after having spent three years at Gateshead, his request to be freed for evangelism full-time was refused yet again, and Booth resigned from the ministry of the Methodist New Connexion.

Soon he was barred from campaigning in Methodist congregations, so he became an independent evangelist. His doctrine remained much the same, though; he preached that eternal punishment was the fate of those who do not believe the Gospel of Jesus Christ and the necessity of repentance from sin, and the promise of holiness. He taught that this belief would manifest itself in a life of love for God and mankind. Eventually, the Booths' children became involved in the ministry.

In 1865 Booth was in the East End of London, preaching to crowds of people in the streets. Outside The Blind Beggar public house some missioners heard him speaking and were so impressed by his preaching that they invited him to lead a series of meetings they were holding in a large tent.

The tent was set up on an old Quaker burial ground on Mile End Waste in Whitechapel. The first of these meetings was held on 2 July 1865. To the poor and destitute of London's East End Booth brought the good news of Jesus Christ and his love for all men.

Booth soon realised he had found his destiny, and later in 1865 he and his wife Catherine opened 'The Christian Revival Society' in the East End of London, where they held meetings every evening and on Sundays, to offer repentance, salvation and Christian ethics to the poorest and most needy, including alcoholics, criminals and prostitutes. The Christian Revival Society was later renamed The Christian Mission.

Slowly The Christian Mission began to grow but the work was difficult and Booth would "stumble home night after night haggard with fatigue, often his clothes were torn and bloody bandages swathed his head where a stone had struck", wrote his wife. Evening meetings were held in an old warehouse where urchin threw stones and fireworks through the window. Outposts were eventually established and in time attracted converts, yet the results were discouraging. The Christian Mission was just one of about 500 charitable and religious groups trying to help the poor and needy in London's East End.

Booth and his followers practiced what they preached and performed self-sacrificing Christian and social work, such as opening “Food for the Million” shops (soup kitchens), not caring if they were scoffed at or derided for their work.

The name The Salvation Army developed from an incident in May 1878. William Booth was dictating a letter to his secretary George Scott Railton and said, "We are a volunteer army." Bramwell Booth heard his father and said, "Volunteer, I'm no volunteer, I'm a regular!" Railton was instructed to cross out the word "volunteer" and substitute the word "salvation". The Salvation Army was modelled after the military, with its own flag (or colours) and its own music, often with Christian words to popular and folkloric tunes sung in the pubs. Booth and the other soldiers in "God's Army" would wear the Army's own uniform, 'putting on the armour,' for meetings and ministry work. He became the "General" and his other ministers were given appropriate ranks as "officers". Other members became "soldiers".

Though the early years were lean ones, with the need of money to help the needy an ever growing issue, Booth and The Salvation Army persevered. In the early 1880s, operations were extended to other countries, notably the United States, France, Switzerland, Sweden and others, including to most of the countries of the British Empire: Australia, Canada, India, South Africa, New Zealand, Jamaica, etc.

Often the beginnings in other countries occurred through "salvationist" activities by non-officers who had emigrated. With some initial success they would contact London to 'send officers.' In other cases, like in Argentina, a non-salvationist told Booth that there were thousands of British people there who needed salvation. The four officers sent in 1890 found that those British were scattered all over the pampas. But the missionaries started ministry in Spanish and the work spread throughout the country - initially following the rail-road development, since the British in charge of building the rail-roads were usually sympathetic to the movement.

During his lifetime, William Booth established Army work in 58 countries and colonies, travelling extensively and holding, "salvation meetings."

Booth regularly published a magazine and was the author of a number of books; he also composed several songs. His book In Darkest England and the Way Out not only became a best-seller after its 1890 release, it set the foundation for the Army's modern social welfare schemes. It compared what was considered "civilized" England with "Darkest Africa" - a land then considered poor and backward. What Booth suggested was that much of London and greater England after the Industrial Revolution was not better off in the quality of life than those in the underdeveloped world.He proposed a strategy to apply the Christian Gospel and work ethic to the problems. The book speaks of abolishing vice and poverty by establishing homes for the homeless, farm communities where the urban poor can be trained in agriculture, training centres for prospective emigrants, homes for fallen women and released prisoners, aid for the poor, and help for drunkards. He also lays down schemes for poor men’s lawyers, banks, clinics, industrial schools and even a seaside resort. He says that if the state fails to meet its social obligations it will be the task of each Christian to step into the breach. However, Booth was not departing from his spiritual convictions to set-up a socialist or communist society or sub-class, supported by people forced to finance his plans; Booth's ultimate aim was to get people "saved."

Booth asserts in his introduction,

I have no intention to depart in the smallest degree from the main principles on which I have acted in the past. My only hope for the permanent deliverance of mankind from misery, either in this world or the next, is the regeneration or remaking of the individual by the power of the Holy Ghost through Jesus Christ. But in providing for the relief of temporal misery I reckon that I am only making it easy where it is now difficult, and possible where it is now all but impossible, for men and women to find their way to the Cross of our Lord Jesus Christ.

It was asserted in some circles that In Darkest England was actually written by the crusading journalist, W.T. Stead, who, in his own words, acted as a "literary hack" for the General when Mrs. Booth lay dying. However, this assumption was swiftly dismissed by Stead some years later, declaring that, "The idea of Darkest England.. was the General's own. My part, of which I had no wish to speak.. was strictly subordinate throughout.

In Darkest England and the Way Out was reprinted several times and lately in 2006.

During its early years The Salvation Army faced a great deal of opposition, especially from those in the alcohol-selling industry who were concerned that the activities of Booth and his followers would persuade the poorer classes to stop drinking. One organisation opposed to Booth and The Salvation Army was the Skeleton Army, a diffuse group, particularly in Southern England, that opposed and disrupted the Salvation Army's marches against alcohol from the early 1880s until about 1892. Clashes between the two groups lead to the deaths of several Salvationists and injuries to many others. During 1882 alone 662 Salvation Army soldiers were assaulted: 251 of them were women and 23 of them were under fifteen years of age.

Other accusations centred around the fact that Booth appointed his own children to posts for which others were better qualified, leading to claims that The Salvation Army was a Booth family-business. For example, he appointed his daughter Emma Booth as the Principal of the Officers' Training Home, The Salvation Army's first training school for women when she was just 19. Others believed that Booth was creating a dynasty, as was suggested by the fact that he insisted that his sons-in-law added 'Booth' to their own names (see Frederick Booth-Tucker and Arthur Booth-Clibborn). This was further borne out when Booth appointed his son, Bramwell Booth, as his successor as General in his will.

The press was often hostile to Booth and The Salvation Army as well because their methods and message were widely misinterpreted. The Army's motto ‘Blood & Fire’, which had deep theological meaning representing the saving ‘blood of Jesus’ and the sanctifying ‘fire of the Holy Spirit’, was erroneously thought to mean the blood of sinners and the fire of hell. There was also suspicion about the Army's motives, with Booth often portrayed as a charlatan only out to make money.

The Church of England was at first also extremely hostile to the activities of Booth and The Salvation Army. The philanthropist, politician and evangelist Lord Shaftesbury even went so far as to describe Booth as the "Anti-Christ". One of the main complaints against Booth was his "elevation of women to man's status". Many found him dictatorial and hard to work with. Some of his own children denounced him as their leader and turned their backs on The Salvation Army, including his daughter Kate Booth and his sons Herbert and Ballington Booth, the latter founding a separate organization, the Volunteers of America with himself as 'General'. The evangelist Rodney "Gipsy" Smith left him because of his rigidity and D.L. Moody would not support him because he felt there was a threat to the local church. But no one could deny his compassion for the sufferings of his fellow man.

Opinion of the Salvation Army and William Booth eventually changed to that of favour. In his later years, he was received in audience by kings, emperors and presidents, who were among his ardent admirers. Even the mass media began to use his title of 'General' with reverence.

In 1899, Booth suffered from blindness in both eyes, but with a short rest, was able to recover his sight. In 1904 he took part in a 'motorcade' when he was driven around Great Britain, stopping off in cities, towns and villages to preach to the assembled crowds from inside his open-top car. In 1906 Booth was made a Freeman of the City of London, and was granted an honorary degree from the University of Oxford. In 1902 he was invited to attend the coronation of King Edward VII.

His last visit to the United States was made in 1907, and in 1909 he embarked on a six month motor tour of the United Kingdom. During this tour he discovered he was blind in his right eye and the sight in his left eye was dimmed by cataracts. The rest of the tour had to be cancelled. On 21 August 1909 a surgeon at Guy's Hospital removed his right eye. Despite this setback, in 1910 Booth campaigned in Holland, Denmark, Germany, Switzerland and Italy. On his return to England he embarked on his seventh and last motor tour.

William Booth was 83 years old when he died (or was Promoted to Glory) at his home in Hadley Wood, London. At the three day lying in state at Clapton Congress Hall 150,000 people filed past his casket. On 27 August 1912 Booth's funeral service was held at London’s Olympia where 40,000 people attended, including Queen Mary, who sat almost unrecognised far to the rear of the great hall.

The following day Booth's funeral procession set out from International Headquarters. As it moved off 10,000 uniformed Salvationists fell in behind. Forty Salvation Army bands played the ‘Dead March’ from Handel’s Saul as the vast procession set off. He was buried with his wife Catherine Booth in the main London burial ground for 19th century non-conformist ministers and tutors, the non-denominational Abney Park Cemetery in Stoke Newington.

In Booth's honour, Vachel Lindsay wrote the poem, "General William Booth Enters Into Heaven". Charles Ives, who had been Evangeline Booth's neighbour, set the poem to music. In 1990 a diesel locomotive in the British Rail fleet was named 'The William Booth'.

The William Booth rose, developed by Agriculture and Agri-Food Canada, was named in his honour. The William Booth Memorial Training College in Denmark Hill, London, the College for Officer Training of The Salvation Army in the United Kingdom, is named after him, as is the William Booth Primary School in his native Nottingham.

William Booth and Catherine Mumford were married on 17 June 1855 at Stockwell New Chapel, Surrey. They had eight children:

• Bramwell Booth (8 March 1856 – 16 June 1929)
• Ballington Booth (28 July 1857 – 5 October 1940)
• Kate Booth (18 September 1858 – 9 May 1955)
• Emma Booth (8 January 1860 – 28 October 1903).
• Herbert Booth (26 August 1862 – 25 September 1926)
• Marie Booth (4 May 1864 – 5 January 1937)
• Evangeline Booth (25 December 1865 – 17 July 1950)
• Lucy Booth (28 April 1868 – 18 July 1953)

File:William booth birthplace.JPG|Plaque on the house in Nottingham in which William Booth was born on 10 April 1829File:William booth statue.JPG|"William Booth" statue at his birth place in NottinghamFile:Reverend William Booth.jpg|Reverend William Booth, General of the Salvation ArmyFile:Abney park booth.jpg|Memorial to William and Catherine Booth in Abney Park CemeteryFile:General William Booth memorial (Battery Park, New York) 01.jpg|General William Booth memorial in Battery Park, New York City