He attended the public schools and graduated from Middlebury College in 1807 with fellow classmates Daniel Azro Ashley Buck and Stephen Royce. He studied law and was admitted to the bar in 1810.
He served as a U.S. Representative from Vermont and the seventeenth Governor of Vermont. He began the practice of law in Middlebury, Vermont. Slide married Abigail Foot Slade on February 5, 1810 in Middlebury.
They had eight children. William Slade was a Democratic-Republican presidential elector in 1812 and 1820. Slade engaged in editorial work.
He established and was editor of the Columbian Patriot from 1814 to 1816 and maintained a book store and printing office. He was Vermont Secretary of State from 1815 to 1822, Judge of the Addison County Court from 1816 to 1822, and Clerk in the US State Department in Washington, D.C. from 1823 to 1829. Slade was elected as an Anti-Masonic candidate to the Twenty-second Congress to fill the vacancy caused by the death of Rollin C. Mallary.
He was reelected as an Anti-Masonic candidate to the Twenty-third and Twenty-fourth Congresses and as a Whig candidate to the Twenty-fifth, Twenty-sixth and Twenty-seventh Congresses, serving from November 1, 1831 to March 3, 1843. While a Representative, he reacted to the first attempt to introduce the Twenty First Rule, which temporarily institutionalized the United States House of Representatives' refusal to discuss slavery. Slade launched into an immediate filibuster, which delayed the passage of the bill by several days.
Slade was the reporter of decisions of the Vermont Supreme Court in 1843 and 1844. He served as Governor of Vermont from 1844 to 1846, defeating Democratic nominee Daniel Kellogg in both of his elections for one year terms. During his tenure, public schools were successfully reorganized.
After leaving office, Slade was corresponding secretary of the Board of National Popular Education from 1846 to 1859, which he co-founded with Catharine Beecher. The Board worked to place female teachers in schools in western United States. Slade died in Middlebury, Vermont on January 18, 1859, and is interred at West Cemetery in Middlebury.
Member United States House of Representatives from Vermont, 22d-27th congresses, November 1, 1831-March 3, 1843.
Married Abigail Foote, February 5, 1810, 9 children.