Ellen Costello is retiring as head of U.S. operations for Bank of Montreal.

The $555 billion-asset Canadian company announced Tuesday that Costello plans to retire as its U.S. country head and as president and chief executive of its BMO Financial unit in Chicago.

Chris Begy, the chief auditor, is set to succeed Costello in her roles, the company said in a press release. He takes the reins Aug. 1.

Costello, 58, joined Bank of Montreal in 1983 and has held leadership positions in Toronto, New York and Hong Kong. In 2006 she became the first woman to lead the company's Harris Bank unit. In 2011 Harris acquired the $52 billion-asset Marshall & Ilsley in Milwaukee for $4.1 billion and the combined institution was named BMO Harris Bank. At the same time, Costello's role was expanded to U.S head, where she oversaw governance and regulatory matters for all of the Canadian company's U.S. businesses.

Mark Furlong remains president and CEO of BMO Harris Bank.

At $95.3 billion in assets, BMO Harris is the largest bank based in Illinois, according to data from the Federal Deposit Insurance Corp. The integration of M&I has been bumpy, including some technical glitches that left customers without access to their accounts. The hiccups helped contribute to a 55-point drop in the company's rating in J.D. Power and Associates 2013 U.S. Retail Banking Satisfaction Study. The bank had a rating of 716, compared with a national average of 763.

Meanwhile, on April 29, the bank entered into formal agreements with state and federal regulators that call for itto strengthen its compliance with the Bank Secrecy Act and anti-money-laundering laws.

Bank of Montreal's U.S. personal and commercial business — essentially BMO Harris Bank — reported earnings of $152 million for the three months ended April 30, up 6.29% from the same period a year earlier.

Costello has been honored numerous times by American Banker as one of the Most Powerful Women in Banking. In 2012 she was ranked seventh.

"Ellen has long been a trailblazer, helping the bank break new ground," Bill Downe, the president and chief executive of Bank of Montreal, said in a press release. "Under her leadership in the U.S., she drove positive results in market share and financial performance and led our efforts in doubling the U.S. retail footprint."

Outside of the bank, Costello is the chairman of United Way of Metropolitan Chicago and sits on the board of several organizations including the Chicago Council on Global Affairs, Chicago Symphony Orchestra and the Financial Services Roundtable.

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