He was the author of several books, including Diseases of the Rectum and Colon and their Surgical Treatment (1923), The Origin of Cancer (1934), and After Us, or the World as it Might Be (1936).

He completed his clinical training in 1899 at St George's Hospital, London, and in 1904 was appointed Hunterian Professor at the Royal College of Surgeons.

During the First World War he operated at King Edward VII's Hospital Sister Agnes, where he treated mainly gunshot wounds affecting the colon, rectum and anus.

[5] Subsequently, he gained admission to Gonville and Caius College, Cambridge, passing the first part of the Natural Sciences Tripos in 1897 with second-class honours.

[3][7] The following year he was Hunterian Professor at the Royal College of Surgeons, where he spoke on the physiology and treatment of surgical shock and collapse, drawing on his book After-treatment of Operations (1903), which ran into four editions and was translated into a number of languages including Arabic.

Lockhart-Mummery became a significant name on the list, probably carried out more operations at the hospital than any other surgeon there, and treated mainly gunshot wounds affecting the colon, rectum and anus.

[10] Among officers he treated was Major Horace Sewell, who had been injured in May 1915 and required shell casing removing from the thigh.

[13] In 1925 his systematic study of people and their families demonstrated a genetic explanation for the association between people with multiple bowel polyps and bowel cancer, 20 years after polyposis was first suggested to be a familial condition by William Harrison Cripps of St Bartholomew's Hospital.

[7] Lockhart-Mummery was a co-founder and key figure in the 1923 British Empire Cancer Campaign, in which he remained active for the rest of his life and which slanted more clinically than the rival scientific Imperial Cancer Research Fund, who had their own significant figures including Walter Morley Fletcher and Frederick Gowland Hopkins.

[7] In the book, he accused "sloppy sentiment" as preventing this method and argued that the nation would have to wait for an "autocratic government" to enforce it for the advantage of men.

[7] Such theories were thought of as radical at that time, considered so even by his friend Lord Horder, president of the Eugenics Society.

[3] From his first marriage to Cynthia in 1915, he had two sons, Hugh Evelyn Lockhart-Mummery, a surgeon who succeeded him at St Mark's Hospital and was later Serjeant-Surgeon to The Queen and knighted;[3][22] and Robert Desmond.

[3] He received an obituary in the British Medical Journal,[24] and biographical profiles in the RCSE's Plarr's Lives of the Fellows,[3] and in Diseases of the Colon & Rectum.