Seattle Mayor Ed Murray announced his decision at a city hall news conference.
O'Toole must be confirmed by the city council. But Murray's decision appeared to be a popular one in many quarters with praise from U.S. Attorney Jenny Durkan and the head of the Seattle chapter of the American Civil Liberties Union.
If O'Toole becomes the first woman to serve as the Seattle police chief, she will be taking over a department hit by scandals.
"Nobody dislikes rogue cops more than good cops," O'Toole said at the news conference. "If people make honest mistakes we'll stand by them."
O'Toole joined the Boston PD in 1979 as a patrol officer. She held top public safety jobs in Massachusetts and served on the Patten Commission, which created the Police Service of Northern Ireland, before being named the head of Boston's department in 2004.
In 2006, O'Toole moved to Ireland to head a three-person inspectorate examining the Garda, the country's national police force. More recently, she has served as the federal monitor for the New Haven, Conn., department.
Capt. Eric Sano, who served on Seattle's search committee, said O'Toole's "breadth of experience is incredible."
"She is a strong leader and a good communicator, and she's no-nonsense," he said.