[1] After studying medicine at King's College, London, Henry Cayley joined the Indian Medical Service in 1857 as an Assistant Surgeon.

After holding the post of Civil Surgeon at Burdwan and then Howrah, he was appointed joint commissioner at Ladakh in 1867.

[3] In December 1867 a newspaper commented that "the reported assassination of Assistant-Surgeon H Cayley, the British representative at Ladak, was without foundation; and some very interesting notes by him respecting that remote region have just been published.

[3] In 1889 the Professor of Military Medicine at the army medical school at Netley Hospital fell ill, and Henry Cayley was invited to step in.

[6] In 1891, he was made Honorary Surgeon to Queen Victoria, and he retained this position when Edward VII came to the throne.

He was granted the temporary rank of colonel in April 1900,[9] as he was sent out in charge of the Scottish National Hospital at Kroonstad in the Orange Free State.