Before becoming president of the Sinn Fein Party from 1983 to 2018, Adams was jailed in the early 1970s under a terrorism law for being a member of the Irish Republican Army, something he long denied. He twice tried to escape, in 1973 and 1974, and was later convicted for the attempts.
At his most recent hearing last fall, Adams argued his imprisonment at HM Prison Maze in County Down, Northern Ireland, was unsafe and hadn't been "personally considered" by then-Northern Ireland Secretary of State Willie Whitelaw.
Wednesday, the British high court agreed in a 14-page ruling.
"Mr. Adams's detention was unlawful, hence his convictions of attempting to escape from lawful custody were, likewise, unlawful," Judge Lord Kerr said at Wednesday's hearing. "The appeal is therefore allowed and his convictions are quashed."
"I have no regrets about my imprisonment except for the time I was separated from my family," Adams said after the decision. "However, we were not on our own. It is believed that around two thousand men and women were interned during [the prison's] four and a half years of operation."
Adams, 71, had been granted an appeal but the lower Northern Ireland Court of Appeals dismissed his claim in January 2018 before it went to the British Supreme Court.
Adams spent more than a decade as a lawmaker in Belfast. Most recently, he spent nine years representing the constituency of Louth in Ireland before leaving office in February.