Seth Williams was born on March 22, 1822 in Augusta, Maine. After obtaining his early education at the local academy, he received an appointment to West Point and was graduated in 1842. After some routine garrison duty, he served through the entire Mexican War as aide-decamp to General Robert Patterson and was brevetted captain for gallantry at Cerro Gordo. He was the "efficient and favorite" adjutant of the Military Academy from 1850 to 1853, and in the latter year formally transferred to the adjutant general's department, where he served the rest of his life. He became major in August, 1861, and brigadier general of volunteers on September 23. From then until March, 1864, Williams was adjutant general of the Army of the Potomac, performing his duties in an eminently satisfactory manner on the successive staffs of such opposed personalities as George B. McClellan, Ambrose E. Burnside, Joseph Hooker, and George G. Meade. When U. S. Grant as general-in-chief chose to make his headquarters with the Army of the Potomac, shortly before the commencement of the Richmond campaign, he selected Williams to be his inspector general, a capacity in which he served until February 9, 1866. He was then assigned as adjutant general of the Military Division of the Atlantic, with headquarters at Philadelphia. Meantime he had been brevetted through grades to that of major general in both the regular and volunteer services. (He won the brevet of major general of volunteers as early as August, 1864.) Toward the end of February, 1866, he became ill and went to his sister's home in Boston, where he died on March 23 of "congestion of the brain." His body was taken to Augusta for interment in Forest Grove Cemetery.
Reference: Generals in Blue. Lives of the Union Commanders by Ezra J. Warner. Louisiana State University Press. Baton Rouge.