James Thacher spent most of his life as a practicing physician; however, today his reputation depends on his writings. Few American diaries of the war equal or surpass his as works of literature, and none of these has the scope of his. Thacher is also known for his writings about medical practice in early America. His American Medical Biography (1828) was the first major American medical reference work and is still an important historical source.

Although descended from distinguished ancestors who had first settled in America in 1635, Thacher's father, John Thacher, was a farmer of modest means. He probably provided his son only a modest basic education before apprenticing him at the age of sixteen for five years to Dr. Abner Hersey of Barnstable, Massachusetts. Thacher's American Medical Biography describes Hersey as "a mere compound of caprice and whim" who "oftener chastened by his frowns than cheered by any...