Irish presenter Liam McKenna has tweeted an image of Walsh stepping off an aeroplane in Kansas City at night as he prepared to join X Factor judges Britney Spears, LA Reid and Demi Lovato.
Cowell has apparently fallen ill with a bout of bronchitis, which has rendered him unable to fulfill his judging duties this weekend.
Walsh jetted to the US after attending auditions for the UK version of The X Factor in Manchester on Thursday (June 7).
An informant previously disclosed details of Cowell choosing Walsh as his replacement, saying: "Simon said [his illness] was an emergency and Louis was the man for the job.
"He said he'd sorted out a private jet for him and he was to get on it as soon as he finished up in Manchester."
The Irish music manager has judged The X Factor UK since 2004, with the exception of a single season where he was replaced by choreographer Brian Friedman for part of it.
The X Factor USA returns to Fox this fall.
Photo gallery - The X Factor USA judges at season two auditions: