Manziel was available to the AAF after the Montreal Alouettes of the CFL released him earlier this month and then said he could not play for any other team in its league.
On Saturday, ESPN's Adam Schefter reported the former Browns QB has signed with the up-start Alliance of American Football League. Manziel was released by the CFL on February 27, citing Manziel had "contravened the agreement which made him eligible to play in the league".
The 2012 Heisman Trophy victor at Texas A&M had his contract with the CFL terminated this offseason. He was awarded through the AAF's waiver system to the Express, who have a league-worst 1-5 record. The Canadian league said he violated his agreement with it, but has offered no specifics. The Express next plays vs. Birmingham on March 24 in Memphis. The Express lost to the Salt Lake Stallions 22-9 in Week 6, losing quarterback Zach Mettenberger to an apparent ankle injury on their first offensive play.
The statement from the Alliance said that Manziel "will report to Memphis tomorrow, where we all believe he'll benefit from the coaching and mentorship of Mike Singletary".
Manziel spent the 2018 season in the CFL with the Hamilton Tiger-Cats and the Montreal Alouettes, where he appeared in eight games, throwing for five touchdowns and seven interceptions.
He was taken with the No. 22 overall pick in the 2014 National Football League draft by Cleveland.
The league's due diligence included an assessment by an independent expert on domestic violence and a head-to-head meeting between Manziel and Ambrosie.
Manziel entered rehab for alcohol and drug abuse in 2015 while still a member of the Browns.
The AAF signed Manziel to a player contract, announcing the news during the broadcast of Saturday's Arizona Hotshots-Orlando Apollos game.