William studied law under the direction of his cousin, Gov. David Campbell of Abingdon, Virginia and attended the lectures of Henry St. George Tucker.
Campbell began the practise of his profession at Carthage, Tennessee.
In his family there was a tradition of courageous military service. Paternal and maternal ancestors had distinguished themselves in colonial wars and in the American Revolution. In 1836 Campbell volunteered for the Seminole War and fought with gallantry as a captain of a company in a regiment commanded by Col. William Trousdale. In the following year he defeated Trousdale in a campaign for the lower house of Congress, despite the fact that the latter had the support of ex-president Jackson.
After six years in Congress he voluntarily retired to private life. He was called from retirement by the Mexican War, when he was elected to command the 1st Regiment of Tennessee Volunteers. At Monterey, at Vera Cruz, and at Cerro Gordo, he and his men of the "Bloody First" fought with a courage and success that made him a popular hero. In 1851 he was the gubernatorial candidate of the Whigs, and again William Trousdale was his Democratic opponent. Trousdale's chief issue was condemnation of the compromise measures of 1850. Campbell championed them and denounced resistance to the laws of the United States. The Whig papers carried as their campaign slogan Campbell's cry at Monterey, "Boys, Follow Me!" and Campbell was elected.
At the end of his term he voluntarily retired again to private life, made his home in Lebanon, and became president of the Bank of Middle Tennessee. The final phase of his life began with the fateful presidential campaign of 1860. He gave his support to John Bell, the Union candidate; he strenuously opposed the secession of Tennessee; and when that act had been accomplished and even John Bell had given allegiance to the Confederacy, he remained the most distinguished of those few in Middle Tennessee who still remained loyal to the Union. From the Confederate authorities he refused offers of high military command. For a brief period he accepted a brigadier-generalship in the Union army. As a "conservative unionist" he worked for the return of Tennessee to the Union. In 1865 he was elected to the lower house of Congress, and when finally seated he gave his support to the conservative policies of President Johnson, but his health had long been failing and he soon died.
Campbell died on August 19, 1867 at his family home of Camp Bell. He is interred at Cedar Grove Cemetery in Lebanon, Tennessee.
Campbell was a member of the newly formed Whig party which for almost twenty years was to be the dominant party in his state.
He served as Governor of Tennessee from 1851 to 1853, and was the state's last Whig governor. He also served four terms in the United States House of Representatives, from 1837 to 1843, and from 1866 to 1867.
During the Mexican-American War, Campbell commanded the First Regiment Tennessee Volunteers, known as the "Bloody First" for its high casualty rate. At the outbreak of the American Civil War, Campbell opposed secession, and briefly served as a general in the Union Army.
He was a Member of the Tennessee State House of Representatives, State Court Judge and Governor of Tennessee.
He was a man of courage and unquestioned integrity, who commanded the respect of those who knew him.
In 1835 he married Frances Owen.