James Williams Forsyth was born in Maumee, Ohio, on August 26, 1834, to James Henry Forsyth and Charlotte Templeton Jackson. After graduating from West Point in 1852, he served at Bellingham Bay and Camp Pickett in Washington Territory from 1856 through 1861. During the Civil War, Forsyth served on the staff of Major General McClelland as an inspector general major and a provost marshal general. He saw action as a major in the 10th Cavalry. When Philip H. Sheridan was given command of a cavalry corps in 1864, Forsyth joined him as his chief of staff. Forsyth was involved in many key battles and distinguished himself in the Shenandoah Valley and Richmond campaigns, for which he was brevetted brigadier general.
From 1866-1867, Forsyth commanded a cavalry brigade and acted as assistant inspector general of the Department of the Gulf. From 1869 to 1873, he was Sheridan's aide-de-camp in the Division of the Missouri, and accompanied him to Europe as an observer during the Franco-Prussian War. Forsyth returned to frontier duty until 1886, when he was promoted to colonel of the 7th Cavalry. The following year he was given command of Fort Riley, Kansas. There he established a school for cavalry and light infantry. In 1890, Forsyth commanded the troops which slaughtered the remnants of the Sioux Nation at Wounded Knee, South Dakota. Though Forsyth was initially relieved of his position because of outrage over the massacre, the Secretary of War exonerated him and reinstated his command. He was further promoted to brigadier general in 1894 and major general in 1897.
Forsyth married Elizabeth Dennison in 1867; they had four children. Forsyth died in Columbus, Ohio, on October 24, 1906.