As of 2023[update], St. Mary's Bank has 11 branches, all in New Hampshire: five in Manchester, two in Nashua, and one each in Hudson, Londonderry, Milford and Portsmouth.

Marie Church in Manchester, began organizing a new financial institution with the goal to help the city's primarily Franco-American millworkers save and borrow money.

He sought assistance from Alphonse Desjardins, who had organized several credit unions in Quebec, and from attorney Joseph Boivin, who volunteered his time and home as the first branch.

[11] French-speaking millworkers were the primary customers of St. Mary's, including children, who often had accounts at the credit union to deposit their wages from working in the mills.

In 1970, St. Mary's Bank built and moved into its present main office at McGregor Street in Manchester.

[22] Within New Hampshire, the credit union significantly changed the economic prospects of Franco-American immigrants, who previously struggled to access the banking system.