Austin Brown told ESPN that Mitchell could have waited until next year and added another year and $80 million to his deal but wanted to commit to a long-term deal as soon as possible.
Mitchell's contract includes a player option for the 2030-31 campaign, according to the report.
Mitchell, who joined the Cavs in a 2022 off-season trade from Utah, said after being eliminated from the playoffs in May by the eventual champion New York Knicks that he had "unfinished business" trying to bring another NBA crown to Cleveland.
James led the Cavaliers to the 2016 NBA title after winning two titles with Miami after leaving the Cavaliers in 2010. He left again for the Los Angeles Lakers in 2018 and won a title with them in 2020.
The four-time NBA Most Valuable Player, however, is a free agent and has said he will not return to the Lakers, with his hometown Cavaliers, Philadelphia, Miami and Golden State among those listed as favourites as landing spots for the 41-year-old forward.
Mitchell, a 29-year-old guard preparing for his 10th NBA season, averaged 27.9 points, 5.7 assists, 4.5 rebounds and 1.5 steals a game last season, when the Cavs reached the Eastern Conference finals before being swept 4-0 by New York.
The Cavaliers have reached the playoffs each of the past four seasons with Mitchell on the club after missing the post-season the four prior campaigns.
With Mitchell around for the long term and center Evan Mobley a solid pillar, the Cavs have what it takes to potentially lure James back to his hometown for another run at a title.