In 1983, Farris founded the HSLDA, of which he served as chairman and general counsel. His efforts resulted in a number of court rulings and policy changes favoring home schooling. In 1993, Farris hired Timmy Teepell, then a homeschooled 18-year-old, to run the Madison Project, a political action committee that raised campaign funds for Christian conservatives. Teepell later became a Republican political consultant in both his native Baton Rouge, Louisiana, and Alexandria, Virginia.

He also founded Patrick Henry College, which opened its doors in 2000. According to HSLDA, the college was founded as

"...a Christian institution with the mission of training students through a classical liberal arts curriculum and apprenticeship methodology to impact the world 'for Christ and for Liberty.'"

He held the positions of president and professor of Government from 2000 to 2006. Farris resigned his position as president of HSLDA to take on these new roles. In March 2006, Farris stepped down from the position of president to become chancellor of the college.

As a lawyer, Farris's cases include over 40 reported decisions as lead counsel. These decisions were given by the United States Supreme Court, five U.S. circuit courts of Appeal, seven state Supreme Courts, and five state Courts of Appeal. Farris successfully argued for the petitioner in a religious freedom case, the Supreme Court case Witters v. Washington Department of Services For the Blind in 1985–1986.

In 1993, Farris ran unsuccessfully for Lieutenant Governor of Virginia and was defeated by Democrat Don Beyer 54–46 percent. However, fellow Republicans George Allen and James Gilmore were elected on the same ballot as Governor and Attorney General, respectively. Farris' close connection to conservative leaders like Jerry Falwell of the former Moral Majority, Pat Robertson of the Christian Coalition and Phyllis Schlafly of the Eagle Forum stirred deep-seated feelings about religion and politics. These concerns, inflamed by negative ads by Beyer to portray him even more radically, likely caused alienation of enough moderate voters to cause his defeat.

Along with Mark Meckler, Farris was co-founder of the Convention of States Project, founded in 2013 to encourage a convention to propose amendments to the US Constitution. He served as Senior Fellow for Constitutional Studies for the project's parent organization, Citizens for Self-Governance, and as a member of CSG's Legal Board of Reference.

Alliance Defending Freedom announced that Farris would become its CEO and General Counsel in January 2017.

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