[5] Keen served as a surgeon for the Fifth Massachusetts Militia Regiment and then for the Union Army during the American Civil War.

[10] Keen was the leader of a team of five that performed a secret surgical operation to remove a cancerous jaw tumor on Grover Cleveland in 1893 aboard Elias Cornelius Benedict's yacht Oneida.

Keen and four assisting doctors made their way to the yacht by boat from separate points in New York, with Cleveland and Bryant boarding in the evening for the night before sailing the next morning.

With calm weather and steady waters, the surgery was finished quickly as the ship transited from Long Island Sound during noontime.

[6] On July 5, Cleveland arrived at Gray Gables to recuperate and was fishing in Buzzards Bay by the end of the month.

[13] Keen was a staunch proponent of vivisection and wrote articles attacking the arguments of anti-vivisectionists,[14] some of which were republished in his 1914 book, Animal Experimentation and Medical Progress.

After 1894, he was a foreign corresponding member[clarification needed] of the Société de Chirurgie de Paris, the Société Belge de Chirurgie, and the Clinical Society of London as well as an honorary fellow of the Royal College of Surgeons of England, the Royal College of Surgeons of Edinburgh, the German Society of Surgery, the Palermo Surgical Society, and the Berliner Medizinische Gesellschaft.