Mired in a sex scandal that crippled the governance of Detroit all year, the city's troubled mayor chose yesterday to walk out of office, rather than run the increasing risk of being heaved out.

The tawdry drama of Kwame Kilpatrick, the once-promising 38-year-old mayor of the nation's 11th-largest city, ended in a wood-paneled courtroom when a subdued Mr. Kilpatrick, after months of defiant claims of innocence, meekly pleaded guilty to reduced felony charges and agreed to serve four months in jail and pay up to $1 million in restitution.

"I lied under oath," Mr. Kilpatrick told the court, conceding what growing numbers of Detroiters have suspected for months -- that he covered up an affair with his former chief of staff, Christine Beatty, as part of a lawsuit settlement that cost the city $8.4 million.

Mr. Kilpatrick, the hip-hop mayor first elected amid great fanfare in 2001, is scheduled to leave office no later than Sept. 18. But in a city whose residents are well aware of the steamy details of text messages between Mr. Kilpatrick and Ms. Beatty, widely reported in the media, the mayor is already history.

"I think there is a giant sense of relief in the city and the entire state over the events of today," said professor Larry Dubin of the University of Detroit Mercy School of Law. "This is like opening up an artery and permitting the blood to flow again for the city."

Mr. Kilpatrick will be succeeded by City Council President Ken Cockrel Jr., 42, one of five council members who voted in May to begin ouster proceedings against the mayor. He will serve until a special election is held.

Mr. Cockrel said it was "a very sad day for the city of Detroit, but I think we also have to recognize it's also a day of hope and renewal." At a Detroit news conference, he declared, "Today we may be grieving, but tomorrow we must come together."

In recent weeks, it became clear that time was running out for Mr. Kilpatrick. Business and religious leaders and the city's African-American newspaper said Mr. Kilpatrick should quit. The prurient interest of the scandal made Detroit the butt of late-night talk-show jokes. Sales transactions aimed at balancing the city's budget were stalled. And Wednesday, Michigan Gov. Jennifer Granholm began expulsion hearings, fanning a flurry of activity aimed at a plea bargain. Ms. Granholm halted yesterday's scheduled hearing after Mr. Kilpatrick's plea

Such a scenario did not seem likely seven years ago, when the charismatic 31-year-old state legislator and son of a powerful congresswoman took over the reins of the shrinking city and seemed to energize the city by force of will.