Born in New Hampshire, Robertson enlisted in the U.S. Army in 1838, and served for ten years in Batteries F and H, 2nd U.S. Artillery. Ranking as a quartermaster sergeant during the Mexican-American War, Robertson received a field commission on June 28, 1848. He was promoted to first lieutenant in 1852. In May 1861, Robertson received his captaincy as the army expanded for the Civil War, and was assigned to command Battery B, 2nd U.S. Artillery. Robertson’s Battery B was combined with Battery L early in the war, and he commanded the combined batteries during the Peninsula Campaign in mid-1862.