A town visitor, Pauline Clarke, captured Mosby's affection. After courting her, he moved to her hometown of Bristol, on the Tennessee border. On December 30, 1857 they were married. Their first child, a daughter named May, was born on May 10, 1859. When Virginia followed other Southern states and voted to secede from the Union following Abraham Lincoln's election to the presidency in 1860, Mosby decided to enlist in the Confederate army.
At first Mosby followed a local company of infantry, but quickly transfered to the cavalry corps of Gen. J.E.B. Stuart, and became acquainted with the duties of a scout. Before too long, however, Mosby became anxious to form his own command, that would not be bound by traditional army conventions. In January 1863, Stuart approved Mosby's plan and gave him a few men to begin his operation. Mosby and his partisan rangers were later incorporated into the regular Confederate army. Their primary objective consisted of destroying railroad supply lines between Washington and Northern Virginia, as well as intercepting dispatches and horses and capturing Union soldiers. Mosby's numbers rose from one dozen to a few hundred by the end of the war. Mosby's rank likewise rose steadily; his final promotion to colonel came in January 1865. Gen. Robert E. Lee cited Mosby for meritorious service more often than any other Confederate officer during the course of the war.
Mosby retreated into a self-imposed exile after the war until he acquired his parole from General U.S. Grant (see full text). He settled down in Warrenton, Va. in Fauquier County to re-establish his law practice. Politics, however, called to him. When Grant became president in 1869, Mosby visited him in the White House and offered his support. Mosby publicly backed the Republican in his 1872 re-election bid, and Grant carried Virginia. Under Hayes, Grant's successor, Mosby became a consul to Hong Kong (1878-1885). After returning to the United States, he became active on the lecture circuit and penned his war reminiscences and several other works for magazines and newspapers, spreading his account of his exploits during the war. After a series of physical debilitations, Mosby died on May 30, 1916 at the age of 82.