[1] His mother's family had a long history of achievement in medicine, starting with Nathan Smith, the third graduate of Harvard College's medical department.
[3] After completing medical school, Longcope took a position as the resident pathologist at Pennsylvania Hospital's Ayer Clinical Laboratory, working under the direction of Simon Flexner.
In 1911, he left to become an assistant professor at Columbia University and physician at Presbyterian Hospital; by 1914, he was the medical director.
[3] Longcope served in World War I as an Army physician, his active duty service beginning in August 1917 and continuing through 1919.
[4] Longcope was one of the founding members of the American Society for Clinical Investigation,[5] and served as the organization's president in 1919.
[4] From the earliest phases of his medical education, Longcope had a particular interest in bacteriology and pathological anatomy.
He was one of the researchers on Dimercaprol ("British Anti-Lewisite") during World War II, and also encouraged its postwar civilian use as an antidote to metal poisoning.