Justice Clarence Thomas has become the second-longest serving justice in U.S. Supreme Court history, a milestone that puts him within reach of a record that has stood for more than 50 years. Record-breaking tenure Clarence Thomas could become the longest-serving justice if he remains on the court until May 2028. By: MEGA Thomas, 77, has served on the high court since October 1991. If he remains on the bench until May 20, 2028, he will surpass the late Justice William O. Douglas, whose tenure of 36 years and 209 days remains the longest in Supreme Court history.