An elder half-brother, Joseph Harrison (1771-1869) moved across the Appalachian Mountains to Hardy in Mineral County well before the Civil War.

Despite his father's death, William received a private education suitable to his class, then traveled westward to Winchester in Frederick County, Virginia, where he read law with his brother-in-law Obed Waite.

[3] After admission to the Virginia bar, Harrison crossed the Appalachian Mountains and began his legal practice in Parkersburg in 1819, where Judge Daniel Smith found him qualified.

[6] Harrison opposed secession and attended a peace conference in Washington D.C. in February 1861, which convinced him that many secessionists were motivated by a lust for power and self-aggrandizement.

When Virginia seceded, Gideon D. Camden, the local judge since 1855 (who had previously represented Lewis County in the House of Delegates in 1825, then Harrison and five adjacent counties in the 1850 Constitutional Convention) sided with the secessionists and would be elected to the First Confederate Congress (though he did not serve),[7] and his son Gideon D. Camden Jr. may have organized a Confederate infantry unit.

The Wheeling Convention later appointed Harrison a member of the Governor's Council, where he helped establish the new state's justice system.

In July 1868, Harrison announced his retirement from the bench, intending for this to take effect on January 1, 1869,[11] though he ultimately left the court on September 1, 1868, due to poor health.