"We've got a deal done," Mark Bartelstein told The Associated Press by phone.
Mason, a 6-foot-5 guard, averaged a career-high 9.1 points and 1.7 assists in 80 games last season with the Washington Wizards. He played in a reserve role for all but nine games. In his previous season with the Wizards, Mason averaged 2.7 points.
Bartelstein said the deal would pay Mason "just a little less than" $8 million. He said he expected Mason, 27, to sign with the team on Friday.
Spurs spokesman Cliff Puchalski said the team had no comment.
Mason averaged eight points and one assist in the playoffs.