On October 9, 1819 Samuel McGowan was Born in the Cross Hill section of Laurens District, South Carolina. He was the son of Irish immigrants. His father, who became a prosperous farmer, fulfilled his ambition to send his sons to college. Samuel attended the school of Thomas Lewis Lesly and graduated from South Carolina College in 1841. He then went to Abbeville where he studied law under T. C. Perrin and was admitted to the bar in 1842. He soon was Perrin's partner and became recognized as a popular politician and advocate.

McGowan served for thirteen years as a member of the South Carolina House of Representatives. His civil career was interrupted by the Mexican War. In 1846 he entered the Palmetto Regiment as a private, rose to the rank of Captain, Staff, and for a time served as a volunteer aide-de-camp to Brigadier-General John A. Quitman whose division stormed the Belen Gate. McGowan was complimented for gallantry in action near Mexico City.

McGowan was elected a Major-General of the South Carolina Militia and in 1861 commanded a brigade that captured Fort Sumter in April of that year. He then volunteered for service, acting as aide-de-camp to Brigadier-General Milledge Bonham at First Manassas.

In 1862 he was commissioned as Colonel, 14th South Carolina Infantry, attached to Maxcy Gregg's brigade. As part of A. P. Hill's Light Division, Gregg's brigade fought valiantly in the Seven Days' Campaign of June 25-July 1. At Gaines' Mill, on June 27, McGowan was wounded but remained with his regiment. Hill cited McGowan and the 14th for its performances at Gaines' Mill and Frazier's Farm.