From Wikipedia (Mann Act) :
It is named after Congressman James Robert Mann of Illinois, and in its original form made it a felony to engage in interstate or foreign commerce transport of "any woman or girl for the purpose of prostitution or debauchery, or for any other immoral purpose". Its primary stated intent was to address prostitution, immorality, and human trafficking, particularly where trafficking was for the purposes of prostitution. This is one of several acts of protective legislation aimed at moral reform during the progressive era. In practice, its ambiguous language about "immorality" has resulted in its being used to criminalize even consensual sexual behavior between adults. It was amended by Congress in 1978 and again in 1986 to apply to transport for the purpose of prostitution or illegal sexual acts.
That bit about "... for any other immoral purpose" is kind of odd wording for a law - surely better would be "... or any other illegal purpose", seeing as that is what laws are supposed to be about.
I was all set to rant about how it was mainly used to target black men having sex with white women and was thus part of the Jim Crow laws that ostensibly targeted everybody, but were almost exclusively used against
Before I started ranting, I wanted some ammunition in the form of statistics about disparate application of the Mann Act, and found instead that many white Americans were prosecuted under its auspices, and that rather than being a Jim Crow law, it was more part of a Puritanical response to the somewhat laissez-faire attitude to prostitution in the 19th and early 20th century.
Such worthies as Charlie Chaplin, Chuck Berry, Frank Lloyd Wright, and yes Jack Johnston (1908-1915 World Champion boxer, and black) were charged under the Mann Act. Chaplin was acquitted, Wright's charges were dropped, Johnson and Berry were convicted. Maybe there was disparate impact, even if there wasn't disparate application.
There are moves now surfacing to have President Trump issue a posthumous pardon for Jack Johnson's conviction, and I was all for it, but now that I investigate further, I think I was wrong.
I could get on board if Congress or the courts were to concede that the initial law (pre-1978 amendments) was unconstitutional for vagueness - the term "immoral acts" was apparently not defined. Following that, President Trump should pardon everybody convicted under the law. To single out any one person would be a travesty.