In a unanimous 8–0 ruling (Thurgood Marshall recused himself due to his previous involvement in the case as a U.S. Department of Justice official), the United States Supreme Court reversed the conviction that had been upheld by the Fifth Circuit.

With the escalation of the Vietnam War, the test standards were lowered in November 1965[4] and Ali was reclassified as 1-A in February 1966,[5][6] which meant he was now eligible for the draft and induction into the U.S. Army.

"[7][8][9] Ali appealed his local (Louisville, Kentucky) draft board's rejection of his application for conscientious objector classification.

In early 1967, Ali changed his legal residence to Houston, Texas,[12][13] where his appeal to be reclassified as a Muslim minister was denied 4–0 on February 20.

He was indicted by a federal grand jury on May 8[17] and convicted in Houston on June 20 of the criminal offence of violating the Selective Service laws by refusing to be drafted.

The Eugene Register-Guard, reporting on the Court's record, cited "...the boxer's beliefs 'are surely no less religiously based' than those in previous cases.

Solicitor General when the case began, and the remaining eight justices initially voted 5 to 3 to uphold Ali's conviction.

However, Justice John Marshall Harlan II, assigned to write the majority opinion, became convinced that Ali's claim to be a conscientious objector was sincere after reading background material on Black Muslim doctrine provided by one of his law clerks.