Fletcher Harris Archer was born on February 6, 1817, in Petersburg, one of the youngest of five sons and four daughters of Allen Archer, a prosperous miller, and Prudence Whitworth Archer. He attended school in Petersburg before entering the University of Virginia, where he received his bachelor of law degree on July 3, 1841. He then returned to his native city and established his practice.
On April 2, 1842, Archer was elected captain of the 7th Company, 39th Virginia Militia Regiment. He held that rank in December 1846, when he raised the Petersburg Mexican Volunteers, which became Company E of the 1st Virginia Volunteer Regiment. His was one of the few Virginia units that saw active military service during the Mexican War. The regiment reached Mexico early in 1847 and served on General Zachary Taylor‘s line until the end of the war. By August 1, 1848, the company was back in Petersburg, where Archer resumed his law practice. He married Eliza Ann Eppes Allen and they had one daughter, born shortly before her mother’s death in April 1851.