Turner Ashby was born on October 23, 1828, in Fauquier County. His father, who died when Ashby was young, had fought in the War of 1812, and his grandfather served under George Washington in the Revolutionary War (1775–1783). Ashby, however, had no formal military training. On the eve of the Civil War, he had settled into an unremarkable life as a merchant and farmer in his boyhood home of Markham. (Little is known about these years, and what is available often comes from eulogistic and exaggerated tales told by entranced biographers.)

Ashby first tasted notoriety in 1859 when, as captain of a volunteer cavalry troop, he led his men to Harpers Ferry in the aftermath of the John Brown raid. Two years later, he returned to Harpers Ferry, this time leading a quasi-official force of Virginians who responded to secession by launching a surprise attack on the federal arsenal there. Such was his popularity in the lower Shenandoah Valley that by June he was commissioned lieutenant colonel of the 7th Virginia Cavalry and mustered into Confederate service.

The critical point in Ashby's life and career was the death of his younger brother Richard, who was mortally wounded in a Union ambush near Kelly's Island on the Virginia border with Maryland on June 26, 1861. From then on, according to his overheated admirers, Ashby was driven by a grim vengeance that bordered on bloodlust. Stories of his deeds became legends, fancy became fact. Those stories were not all myths—Ashby thrived and even thrilled in combat—and they became the source of a mesmerizing aura that was all the more powerful because it quelled fears while it idealized hopes. Young men began flocking to him, seeking in Ashby's afterglow something of his cavalier image. To call Ashby the "Knight of the Valley," as many did in 1861, was simultaneously to obscure the brutality of partisan war on the Maryland border and cast it in familial terms as a chivalric defense of home.

By the spring of 1862 Ashby had superseded Angus W. McDonald as colonel and commander of the 7th Cavalry, which thanks to Ashby's aura had grown into a loosely organized and undisciplined collection of twenty-six companies. Moreover, Ashby's cavalry, which operated independently for the first year of the war, was now co-opted into Jackson's Army of the Valley. By and large, Ashby served Jackson well in the latter's illustrious Valley Campaign, a stunning masterpiece of deception, movement, and quick striking that is often credited with discomfiting Union general George B. McClellan's attempt to capture the Confederate capital at Richmond and thus end the war.

Ashby's fame grew as the campaign unfolded, notwithstanding two incidents that speak to his strengths and weaknesses. The first, a serious mistake in reconnaissance preceding the Confederate defeat at Kernstown in March 1862, suggests Ashby's limited mastery of formal military operations. Ashby thought of his duty in far too simple terms: he sought out the enemy and fought them. He was neither an administrator nor a disciplinarian. The second incident, that April, found Ashby at odds with Jackson, who tried to correct those problems by removing Ashby from command so that his disorganized troopers could be properly trained. Ashby reacted to Jackson's impersonal methods by resigning and speaking openly if vaguely about challenging Jackson to a duel. The affair's resolution says much about Ashby's inspirational, personal charisma. Ashby's cavalry would follow no other leader, a fact Jackson recognized by restoring him to command and, according to one observer, "backing square down." Just a month later, and over Jackson's strident objections, Ashby was promoted to brigadier general.

Ashby was killed at the tail end of the Shenandoah Valley Campaign during a skirmish near Harrisonburg that June. His remains were reinterred after the war in the Stonewall Cemetery in Winchester, where, as testament to the ways in which Ashby came to symbolize the Confederate defense of home, he was laid with his brother Richard in one grave.