As of March 2024[update], it was the 15th largest bank in the United States by total assets.

Also in 2002, InvestorLine combined with CSFBdirect to form Harrisdirect and Harris acquired online client accounts of Morgan Stanley Individual Investor Group and myCFO.

[9] On December 17, 2010, Bank of Montreal agreed to purchase Milwaukee-based Marshall & Ilsley Corporation in an all-stock transaction valued at about US$4.1 billion.

In December 2015, the company completed its acquisition of General Electric Capital Corp.'s transportation finance business.

[10] BMO was the bank with the second-most deposits in Chicago by June 2018, with 11.5% market share.

[16] BMO is one of the largest banks in the Midwest with over 600 branches and approximately 1,300 ATMs in Illinois, Iowa, Wisconsin, Indiana, Kansas, Missouri, Minnesota, Arizona, Florida and California.

[18] In December 2018, the bank announced it would move its headquarters to 14 floors of a 50-story office tower adjacent to Chicago Union Station.

In 2017, the bank began construction of a 25-story BMO Tower at North Water and East Wells, adjacent to its current Milwaukee headquarters.

The bank expects to occupy 123,000 square feet (11,400 m2) of the building beginning in December 2019.

In August 2017, BMO sold the 20-story structure it currently occupies to real estate firm Irgens, which plans to remodel and lease it.