In announcing his decision, Dart reiterated that he loved his current job, that it keeps him busy, and that he isn't willing to disrupt his family further with a mayoral run.
"This could not be more of a personal decision," Dart said. "I've always had an interest, but there are too many days in a row I don't see my children awake."
The announcement comes after weeks of speculation that Dart, a high profile public official with a strong support base in the 19th Ward, would take advantage of Mayor Richard Daley's retirement.
Dart said that fundraising would not have been an issue. He struggled with the decision for a week or two.
"I feel I would be a great mayor," said Dart. "But it tears at you."
Dart was considered to be, out of the gate, a frontrunner for mayor and a top contender against unofficial candidate Rahm Emanuel.
Dart said that he doesn't think that staying out of the race will hand a victory to Emanuel.
"I don't believe I'm paving the way for anybody," he said. "I don't even know who's formally in or not."
Dart repeatedly denied he was making a political calculation.
"To be honest with you, I'm not a very good politician," Dart said. "And I like it that way."
Dart and Emanuel did bump into each other at a recent fundraiser and exchanged hellos. Several potential mayoral candidates said in recent weeks that Emanuel had contacted them, furthering speculation that Emanuel was attempting to clear the field of potential challengers.
Dart began his career as a state prosecutor in Cook County. He was appointed to the state Senate in 1991, and was then elected a state representative in 1993. In 2003, Dart became chief of staff to then-Cook County Sheriff Michael Sheahan. Dart then ran for state treasurer in 2002, but lost to incumbent Judy Baar Topinka. Upon Sheahan's retirement, Dart ran for and won his current office.
Dart first came to national attention in 2006 when he suspended foreclosure evictions in Cook County. Dart also made headlines for his policing of the Burr Oak scandal, and for suing Craigslist in 2009 over its erotic services section. The suit was dismissed, though Craigslist voluntarily censored the section this year.