He was the son of an Irish solicitor, and went to Rugby School in 1903 and then in 1907 to the University of Oxford, where he studied history. He entered the Indian Civil Service in 1911 and was posted to Burma in 1912. He had postings at Sagaing and elsewhere. In 1917, the British army raised a Burmese brigade with which Collis went to Palestine, but he saw no action. In 1919, he went on leave and travelled in Europe. In the 1920s he was district commissioner in Arakan. In 1929-1930, a period when relations between Burmese, Indians and British became particularly difficult, he was district magistrate in Rangoon. This period is narrated in his memoir Trials in Burma. He gives special attention to the political trial of J. M. Sen Gupta, mayor of Calcutta, for sedition in impromptu speeches made during a brief visit to Rangoon in 1930; also to two criminal trials which became politically charged because they brought to light underlying attitudes of British merchants and army officers to Burmese people. Collis's judgments were (according to his own analysis) too independent to be pleasing to the then British Government of Burma, arousing the particular disapproval of his superior, Booth Gravely, Commissioner of the Pegu Division. After giving judgment in the last of these trials Collis was hastily moved to the post of Excise Commissioner. After returning to England in 1934, he wrote many books, including Siamese White and Foreign Mud, as well as art and literary criticism. At the age of 65 he turned his hand to painting.

His younger brothers were the writer John Stewart Collis and Robert Collis, a famous doctor and author.

Autobiographies

• The Journey Outward ends 1917 - 18
• Into Hidden Burma 1919 -34
• The Journey Up (Reminiscences 1934-1968)
• Trials in Burma 1930 -31

Biographies

• Siamese White
• Cortés and Montezuma (about the Spanish conquest of Mexico)
• Raffles (Faber, London, 1966; about Stamford Raffles)
• Marco Polo
• The Grand Peregrination - Being the Life and Adventures of Fernão Mendes Pinto
• Nancy Astor - An Informal Biography
• Somerville and Ross - A Biography (Faber, London, 1968; about Edith Somerville and Violet Martin = "Martin Ross")
• Stanley Spencer - A Biography
• Wayfoong - The Hongkong and Shanghai Banking Corporation

Histories

• The Great Within
• The Land of the Great Image - Being Experiences of Friar Manrique in Arakan (Faber & Faber, London, 1943 1st edition )(New York: Alfred A. Knopf, 1943. First American Edition)
• Foreign Mud - Being an Account of the Opium Imbroglio at Canton in the 1830s and the Anglo-Chinese War That Followed, 1946
• The First Holy One
• British Merchant Adventurers
• The Hurling Time
• Last and First in Burma

Fiction

• She Was a Queen
• The Mystery of Dead Lovers (with drawings by Cawthra Mulock)
• Quest for Sita
• Sanda Mala
• The Dark Door

Drama

• The Motherly and Auspicious - Being the Life of the Empress Dowager Tzu Hsi in the Form of a Drama, with an Introduction and Notes
• White of Mergen

Other

• Lords of the Sunset - A Tour in the Shan States. Collis toured the Shan State in Northern Burma in the winter of 1937, meeting the various local rulers, attending a funeral, and following a murder trial.
• Alva Paintings and Drawings
• Lord of the Three Worlds