Mitchell was born in Greenville to Clyde D. Mitchell and Dothenia E. Mitchell. He grew up in a broken household and his father moved to Newark, New Jersey to escape the segregationist practices of the South. Upon completion of high school, Mitchell majored in biology at Fisk University in Nashville, Tennessee and he aspired to be a doctor. After obtaining his undergraduate degree, Mitchell worked on cancer research in Washington, D.C., but while there he enrolled in law school at Howard University. Mitchell returned to South Carolina in 1969 to attend his grandmother's funeral and found a changed atmosphere that provided economic opportunities for blacks.
Choosing to remain in South Carolina, Mitchell practiced law for the Legal Services Agency of Greenville, Inc. He ran for a seat in the South Carolina House of Representatives in 1972, but lost the race. However, two years later in 1974, Mitchell won the seat for District 23. In 1982, Mitchell was indicted on a charge of illegal possession of food stamps when a client of his allegedly gave him food stamps as payment for legal services. A mistrial was declared and the charges were dropped after the jury could not reach a verdict in the case. Mitchell won election to the South Carolina Senate for District 7 in 1984 and served as a chairman of the Senate Corrections and Penology Committee during his tenure.
Mitchell sought to emulate the recent successes of black politicians such as Jesse Jackson and Douglas Wilder by announcing his candidacy in January for the South Carolina gubernatorial election of 1990. He faced a Democratic primary challenge from State Senator Ernie Passailaigue, but Mitchell won easily thanks in large part due to the support of black voters. Mitchell went on to lose the general election to Governor Carroll A. Campbell, Jr. because of his inability to attract support from the white state Democratic Party establishment.
The South Carolina Senate voted in 1995 to expel Mitchell from the Senate by a vote of 38 to 7 because he had violated federal tax laws. It was the only time that a black member had been expelled from the state Senate. Additionally in response to his expulsion, the state Senate proposed a bill that if a member of the legislature resigns or is expelled, they must repay any compensation received. Upon the completion of his 90-day sentence, he lost the special election to fill his vacant seat.
Mitchell established the law firm Theo W. Mitchell and Associates in Greenville to specialize in civil rights, human rights and criminal law. However, the state Supreme Court issued a public reprimand in 2005 to Mitchell because of the use of the word Associates in the law firm's name because there were no other practicing attorneys in his law firm that were licensed in South Carolina. Mitchell also serves on the Board of Directors of the LaRouche Movement's Schiller Institute.
Mitchell married Greta Knight of Pueblo, Colorado and they had three daughters. They are both Life Members of the NAACP and active members of Allen Temple A.M.E. Church in Greenville. Member of Omega Psi Phi fraternity. His grand daughter Angelique Monét is the former Ms. Black South Carolina, a well known multimedia artist in the film and television industry.
- "Mitchell makes history with win: Greenville Democrat to take on Campbell". The State. 13 June 1990. p. 1A, 8A.
- "Mitchell loss not a surprise: Popular incumbent never looked back". The State. 7 November 1990. p. 4A.