Constant was born in Lausanne, Switzerland, to descendants of noble Huguenots who fled France during the Huguenot wars in the early 16th century to settle in Lausanne. He was educated by private tutors and at the University of Erlangen, Bavaria, and the University of Edinburgh, Scotland. In the course of his life, he spent many years in France, Switzerland, Germany, and Great Britain.

He was intimate with Anne Louise Germaine de Staël and their intellectual collaboration made them one of the most important intellectual pairs of their time. He was a fervent liberal, fought against the Restoration and was active in French politics as a publicist and politician during the latter half of the French Revolution and between 1815 and 1830. During part of this latter period, he sat in the Chamber of Deputies, the lower legislative house of the Restoration-era government. He was one of its most eloquent orators and a leader of the parliamentary block first known as the Indepentants and then as "liberals."

Constant died in Paris on December 8, 1830.

One of the first liberal thinkers to go by the name, Constant looked to Britain rather than to ancient Rome for a practical model of freedom in a large, commercial society. He drew a distinction between the "Liberty of the Ancients" and the "Liberty of the Moderns". The Liberty of the Ancients was a participatory, republican liberty, which gave the citizens the right to directly influence politics through debates and votes in the public assembly. In order to support this degree of participation, citizenship was a burdensome moral obligation requiring a considerable investment of time and energy. Generally, this required a sub-society of slaves to do much of the productive work, leaving the citizens free to deliberate on public affairs. Ancient Liberty was also limited to relatively small and homogenous societies, in which the people could be conveniently gathered together in one place to transact public affairs.

The Liberty of the Moderns, in contrast, was based on the possession of civil liberties, the rule of law, and freedom from excessive state interference. Direct participation would be limited: a necessary consequence of the size of modern states, and also the inevitable result of having created a commercial society in which there are no slaves but almost everybody must earn a living through work. Instead, the voters would elect representatives, who would deliberate in Parliament on behalf of the people and would save citizens from the necessity of daily political involvement.

He chastised several of the aspects of the French Revolution and the failures within the social and political upheaval. He stated how the French attempted to apply ancient republic liberties to the modern state. Constant realized that freedom meant drawing a frontier between the area of a person’s private life and that of public authority . He admired the noble spirit of regeneration of the state; however, he stated that it was naïve that writers believed that two thousand years had not wrought some changes in disposition and needs of people. The dynamics of the state had changed: the ancient states’ population paled in comparison to that of the modern countries. He even argued that with a large population man had no role in government regardless of its form or type. Constant emphasized how the ancient state found more satisfaction in their public existence and less in their private. However, the satisfaction of modern peoples occur in their private existence.

Constant’s repeated denunciation of despotism pervaded his critique of French political philosophers Jean-Jacques Rousseau and Abbé de Mably. These writers, influential to the French Revolution, according to Constant, mistook authority for liberty and approved any means of extending the action of authority. Reformers used the model of ancient states of public force and organized the most absolute despotism under the name of Republic. He continued to condemn despotism, citing the paradox of liberty derived from recourse to despotism, and the lack of substance in this ideology.

Furthermore, he pointed out the detrimental nature of the Reign of Terror; the inexplicable delirium. In François Furet’s words, Constant’s “entire political thought” revolved around this question, namely the problem of explaining the Terror. Constant understood the revolutionaries’ disastrous over-investment in the political . The French revolutionaries such as the Sans-culottes were the primary forces in the streets. They promoted constant vigilance and a public person. Constant pointed out how the most obscure life, the quietest existence, the most unknown name, offered no protection during the Reign of Terror. He also stated that each individual added to the number, and took fright in the number that he had helped increase. This mob mentality deterred many and helped to usher in new despots such as Napoleon.

Moreover, Constant believed that in the modern world, commerce was superior to war. He attacked Napoleon's martial appetite on the grounds that it was illiberal and no longer suited to modern commercial social organization. Ancient Liberty tended to be warlike, whereas a state organized on the principles of Modern Liberty would be at peace with all peaceful nations.

Constant believed that if liberty was to be salvaged from the aftermath of the Revolution, then chimerical Ancient Liberty had to be abandoned in favour of practical and achievable Modern Liberty. England, since the Glorious Revolution of 1688, and then the United Kingdom after 1707, had demonstrated the practicality of Modern Liberty and Britain was a constitutional monarchy. Constant concluded that constitutional monarchy was better suited than republicanism to maintaining Modern Liberty. He was instrumental in drafting the "Acte Additional" of 1815, which transformed Napoleon's restored rule into a modern constitutional monarchy. This was only to last for "One Hundred Days" before Napoleon was defeated, but Constant's work nevertheless provided a means of reconciling monarchy with liberty. Indeed, the French Constitution (or Charter) of 1830 could be seen as a practical implementation of many of Constant's ideas: a hereditary monarchy existing alongside an elected Chamber of Deputies and a senatorial Chamber of Peers, with the executive power vested in responsible ministers. Thus, although often ignored in France because of his Anglo-Saxon sympathies, Constant made a profound (albeit indirect) contribution to French constitutional traditions.

Secondly, Constant developed a new theory of constitutional monarchy, in which royal power was intended to be a neutral power, protecting, balancing and restraining the excesses of the other, active powers (the executive, legislature, and judiciary). This was an advance on the prevailing theory in the English-speaking world, which, following the conventional wisdom of William Blackstone, the 18th century English jurist, had reckoned the King to be head of the executive branch. In Constant's scheme, the executive power was entrusted to a Council of Ministers (or Cabinet) who, although appointed by the King, were ultimately responsible to Parliament. In making this clear theoretical distinction between the powers of the King (as head of state) and the ministers (as Executive) Constant was responding to the political reality which had been apparent in Britain for more than a century: that the ministers, and not the King, are responsible, and therefore that the King "reigns but does not rule". This was important for the development of parliamentary government in France and elsewhere. It should be noted, however, that the King was not to be a powerless cipher in Constant's scheme: he would have many powers, including the power to make judicial appointments, to dissolve the Chamber and call new elections, to appoint the peers, and to dismiss ministers - but he would not be able to govern, make policy, or direct the administration, since that would be the task of the responsible ministers. This theory was literally applied in Portugal (1822) and Brazil (1824), where the King/Emperor was explicitly given "Moderating Powers" rather than executive power. Elsewhere (for example, the 1848 "Statuto albertino" of the Kingdom of Sardinia, which later became the basis of the Italian constitution from 1861) the executive power was notionally vested in the King, but was exercisable only by the responsible ministers.

Constant's other concerns included a "new type of federalism": a serious attempt to decentralize French government through the devolution of powers to elected municipal councils. This proposal reached fruition in 1831, when elected municipal councils (albeit on a narrow franchise) were created.

The importance of Constant's writings on the liberty of the ancients has dominated understanding of his work. Constant was, however, no proponent of radical libertarianism. His wider literary and cultural writings (most importantly the novella Adolphe and his extensive histories of religion) emphasized the importance of self-sacrifice and warmth of the human emotions as a basis for social living. Thus, while he pleaded for individual liberty as vital for individual moral development and appropriate for modernity, he felt that egoism and self-interest were insufficient as part of a true definition of individual liberty. Emotional authenticity and fellow-feeling were critical. In this, his moral and religious thought was strongly influenced by the moral writings of Jean-Jacques Rousseau and German thinkers such as Immanuel Kant, whom he read in preparing his religious history.

Constant published only one novel during his lifetime, Adolphe (1816), the story of a young indecisive man's disastrous love affair with an older mistress. A first-person novel in the sentimentalist tradition, Adolphe examines the thoughts of the young man as he falls in and out of love with Ellenore, a woman of uncertain virtue. Constant began the novel as an autobiographical tale of two loves, but decided that the reading public would object to serial passions. The love affair depicted in the finished version of the novel is thought to be based on Constant's affair with Anna Lindsay, who describes the affair in her correspendence (published in the Revue des Deux Mondes, December 1930-January 1931). The book has been compared to Chateaubriand’s Rene or Mme de Stael’s Corinne.