Dr. Walker was born in Fryeburg, Maine, and received his early education at a local academy and Dartmouth College. For health reasons, he traveled in the south for several years and, in 1850, received a medical degree from Harvard. He began his medical practice in Boston under Dr. Charles Stedman, who was the physician at the Boston Lunatic Asylum.
In 1851, Dr. Walker became Superintendent of the Boston Lunatic Asylum and held this position until 1881. He instituted reforms there, including abolishing the practice of placing violent patients in cells. He labored to obtain better accommodations for the insane, but although funds were obtained, the mayor vetoed the project. Dr. Walker later provided medical supervision for the construction of the Danvers State Hospital, which used Walker’s plan for a new Boston Lunatic Asylum.
In 1872, he toured institutions in Europe. During the Civil War, he was appointed Inspector of Hospitals and made a tour of service in the west. Dr. Walker was President of the Association of Medical Superintendents of American Institutions for the Insane (1879–1882).