He introduced electrocardiography to Canada and England and was the first to recognise the relationship between finger clubbing in adults with acquired structural heart disease and infective endocarditis.

He then spent a short time in postgraduate training in the United States before moving to University College Hospital, London, in 1913, where he worked with Sir Thomas Lewis.

[2] Despite the age difference and distance between the two, Sir William Osler supported Cotton's postgraduate activities in England and abroad.

Cotton later wrote, "the gates were opened wide for me to enter; a letter to Lewis from William Osler was all that was required to make life seem at its best".

[3] At the onset of the First World War, Cotton enlisted in the Canadian Expeditionary Force and re-joined Lewis at the Military Hospital, Hampstead, which had been established to carry out research into heart diseases in soldiers.

[3] It was at Hampstead that his path crossed with William Osler, Mackenzie, Clifford Allbutt, Caler, Meakins, Parkinson and Drury.