WASHINGTON — For more than 100 years, Jack Johnson’s legend as the first black heavyweight boxing champion has been undisputed, but his legacy had been tarnished by a racially tainted criminal conviction.

His battles against white opponents, in the ring and outside of it, gave rise to “The Great White Hope” play and movie and he came to be lionized as a barrier breaker.

But the criminal conviction from 1913 that most would find abhorrent today — for transporting a white woman across state lines — haunted Johnson well after his death in 1946 and motivated politicians and celebrities for years to advocate for a pardon, however symbolic.

On Thursday in the Oval Office, Johnson posthumously found an unexpected champion: President Trump.

Although his own record on civil rights has come under question, often harshly, Mr. Trump, flanked by boxing champions and Sylvester Stallone, the actor who brought the case to his attention, signed an order pardoning Johnson.