Reynolds was nominated to the United States Military Academy in 1837 by Senator James Buchanan, a family friend, and graduated 26th of 50 cadets in the class of 1841. He was commissioned a brevet second lieutenant in the 3rd U.S. Artillery, assigned to Fort McHenry. From 1842 to 1845 he was assigned to St. Augustine, Florida, and Fort Moultrie, South Carolina, before joining Zachary Taylor's army at Corpus Christi, Texas, for the Mexican–American War. He was awarded two brevet promotions in Mexico—to captain for gallantry at Monterrey and to major for Buena Vista, where his section of guns prevented the Mexican cavalry from outflanking the American left. During the war, he became friends with fellow officers Winfield Scott Hancock and Lewis A. Armistead.