James played only one game with the Legends, but sources told ESPN.com that he quickly convinced the Mavericks that he can help address the team's lack of veteran leadership in the backcourt after the departure of Derek Fisher last month.
James was scheduled to play with the Legends here in Reno on Monday afternoon in their first game in the D-League's annual 16-team showcase tournament against the Idaho Stampede before signing the 10-day deal.
James, 37, scored 26 points in three quarters Saturday night in his Legends debut against Tulsa.
James earned a call-up from the Chicago Bulls last season in similarly fast fashion after playing strongly in his first game at the 2012 Showcase, which was also held in Reno.
The Mavericks opened a roster spot for a 10-day signing by waiving swingman Chris Douglas-Roberts on Sunday.
After playing for 10 NBA teams, James cleared the D-League waiver process last week to become eligible to sign with the Legends as a free agent.
James arrived in town with no guarantees from the Mavericks, sources said, but the 37-year-old lobbied them to let him audition for a roster spot by playing in a few games with the Legends.
Monday was the first day NBA teams can start signing players to 10-day contracts. The Mavericks moved fast in part because even though James was playing for their D-League affiliate, they held no exclusivity in terms of James' NBA rights.
James averaged 20.3 points and 5.8 assists for the Toronto Raptors in his breakout season of 2005-06 and then negotiated seriously with Dallas in free agency before opting to join the Minnesota Timberwolves.
The Mavericks have been looking to strengthen their options at point guard all season thanks to the ups and downs endured by projected starter Darren Collison and a lack of progress from holdover youngsters Dominique Jones and Rodrigue Beaubois. The Mavs also waived Delonte West in November after employing West for one season and had Fisher on board for only nine games before the team and Fisher mutually agreed to part company before Christmas.
The Mavericks were pleased with Fisher's contributions, sources said, and have missed his veteran know-how during their recent 1-9 slide.