The bank grew rapidly through a series of acquisitions beginning with the First National Bank of Barrington in 1985, State Bank of St. Charles and First National Bank of Batavia in 1988 and Libertyville Federal Savings Bank and Loan and Frankfort Bancshares in 1990.
On December 17, 2010, Bank of Montreal agreed to purchase Milwaukee-based Marshall & Ilsley Corporation in an all-stock transaction valued at about $4.1 billion USD.