Men’s basketball coach Paul Hewitt will lead George Mason in the Atlantic 10 next season. (Toni L. Sandys/The Washington Post)

Less than a year after resisting the Atlantic 10’s overtures, George Mason will announce Monday that it is joining the conference and leaving the Colonial Athletic Association after 28 years, effective July 1, multiple people with knowledge of the situation said Sunday night.

Both the university and Atlantic 10 have scheduled news conferences 45 minutes apart Monday afternoon.

George Mason will make a “special announcement regarding [the] future” of the athletic program, according to a news release. The Atlantic 10 used similar language in advance of its announcement.

GMU Athletic Director Tom O’Connor and university President Angel Cabrera did not respond to messages seeking comment.

However, people familiar with George Mason’s decision confirmed the move. They requested anonymity because they’re not authorized to comment publicly until the formal announcements.

Almost all of the school’s 20 sports are affected, but the decision was driven by men’s basketball, the most prominent sport at a school that does not have a football program. The Atlantic 10 had five teams competing in the NCAA tournament this week; the CAA had one. However, with Xavier, Butler, Temple and Charlotte leaving this summer, the Atlantic 10 needed to replenish its membership.

The switch will reunite the Patriots with longtime nemesis Virginia Commonwealth, which jumped to the Atlantic 10 this season, and forge a local rivalry with George Washington, an Atlantic 10 member since 1976.

Last May, after entering talks with the Atlantic 10, George Mason recommitted to the CAA. “We felt it was in our best interests to stay,” O’Connor said at the time, citing the conference’s history and geographic strength.

The Patriots have been among the CAA’s most successful teams since helping found the league in 1985, winning six tournament championships and advancing to the 2006 NCAA Final Four. But the CAA has fallen on hard times in the past year, losing VCU, a 2011 Final Four participant, to the Atlantic 10 and Old Dominion to Conference USA.

Only seven teams competed in the CAA tournament this month. ODU and Georgia State, which is headed to the Sun Belt Conference, were ineligible because they had announced their intentions to leave the league, primarily for football reasons; and Towson and UNC Wilmington did not meet the NCAA’s minimum academic standards.

The CAA was one of the weakest mid-major basketball conferences in the country this season, and tournament champion James Madison was a No. 16 seed in the NCAA tournament. College of Charleston will join the CAA from the Southern Conference next season.

With VCU and ODU exiting, the league decided to move the conference tournament to 1st Mariner Arena in Baltimore from Richmond Coliseum, starting next season.