Having first been announced in December 2010, the transaction has now been finalized, resulting in the lender becoming the 12th biggest commercial bank in the US.
The agreement has seen BMO - established in 1817 and now serving 12 million customers worldwide - double its stake in the country's market and it will lead to M&I Bank combining with Harris Bank to form BMO Harris Bank NA, which is to be part of BMO Financial Group.
Bill Downe, president and chief executive officer at the group, said the movement will result in the bank setting a new standard for excellence in the sector.
"We are a strong Midwest-rooted, socially-responsible bank that puts customers first and is deeply committed to the communities in which our employees live and work," he added.
By Claire Archer