The deal, whose financial terms have not been disclosed, is subject to regulatory approval.
The transaction is expected to close in the fourth quarter of this year.
The deal will allow the MTIA to leverage Arthur J Gallagher’s resources in the areas of global broking and insurance services operations.
However, the agency would continue to serve the M&T Bank’s footprint.
Being a retail insurance agency, MTIA caters to customers in Northeast and Mid-Atlantic areas of the US.
It offers customised health benefits and property and casualty products.
M&T head of institutional client services business Jennifer Warren said: “When we started to discuss the possibility of this transaction with Gallagher, it became immediately clear that this was the optimal company for both our customers and our employees.
“Our goal was to create a seamless transaction with a world-class company that stresses excellence in everything they do. We found that with Gallagher.”
Early this year, Gallagher acquired the remaining stake in Capsicum Reinsurance Brokers, increasing its ownership from 33% to 100%.
The acquired company was launched to serve as the reinsurance broking arm for Gallagher, eventually becoming the main treaty reinsurance outlet of the Gallagher group of companies.
It March 2020, Gallagher also purchased full-service wholesale insurance broker CMS, bolstering its presence in New York.