The low-cost deposits that come with the deal should fund the Michigan company's effort to become a commercial lender.
The company will snag its first branches in three towns in Connecticut and Rhode Island after buying locations from Webster Financial.
The company will also sell $58 million in deposits to West Gate Bank.
The Justice Department required the sales before signing off on MainSource's pending sale to First Financial in Cincinnati.
The divestitures are necessary to get the Justice Department to sign off on First Financial's $1 billion purchase of MainSource Financial.
Wings Financial Credit Union will also gain about $40 million in deposits and $10 million in loans.
The Tennessee branches are being sold as part of First Horizon's deal to buy Capital.
The company will also gain $260 million in loans and $180 million in deposits from Banner Corp.
The $2.6 billion-asset parent of TBK Bank is acquiring nine branches from Independent Bank Group.
The deal includes $52.6 million in deposits and $13.2 million in loans, and the bank has indicated it plans to close two more locations in Wisconsin in June.
