During his tenure in Congress Koch spoke out against the war in Vietnam and worked for federal aid to mass transit, health care for the aged, and a family assistance program, earning high praise from the liberal-left Americans for Democratic Action.

In September 1977 Edward Koch entered a seven person Democratic primary seeking the mayoral nomination from which he emerged as the winner, beating chief challenger Mario Cuomo. In the heated election campaign which followed attempts were made to smear the unmarried Koch as a homosexual with signs that read: "Vote for Cuomo, not the homo." Ed Koch handily beat the future New York governor with 713,000 votes to Liberal Party candidate Mario Cuomo's 587,000. The dirty politics campaign embittered relations between these two leading New York democrats (which were exacerbated again in 1982 when Koch challenged Cuomo for the gubernatorial nomination). Koch had also struck a responsive chord with Gotham voters in his tough talk about cracking down on crime and the need for fiscal solvency and his combative willingness to stand up to the powerful municipal unions and minority special interests groups.

When Edward Koch took over the mayoralty in January 1978 the city was teetering on the edge of bankruptcy and saddled with a $1 billion deficit. Koch, with the continuing aid of federal loan guarantees and the Municipal Assistance Corporation, bore down on extravagance, pared the city payrolls, cut superfluous and inefficient public services, and brought the city back to fiscal solvency, boasting of a $500 million surplus by 1983. His real talent was to say "no" to excessive expenditures and over-generous settlements with municipal unions. With his clever quips and zany sense of humor, he brought a majority of New Yorkers around to liking his new lean, mean brand of urban austerity.

Koch was reelected mayor in 1981 as a candidate of both the Democratic and Republican parties. Persuaded by New York Post publisher Rupert Murdoch and his own upward ambitions, Koch entered the 1982 Democratic primary for governor and again faced his bitter rival, Mario Cuomo. He lost the primary. Koch may have contributed greatly to his own defeat by an impolitic interview with Playboy magazine, which his foes publicized and in which Koch referred to suburban life as "sterile" and rural life as "a joke" and made fun of pickup truck drivers, the very voters he needed to win the nomination.

Reelected to a third term as mayor in 1985, Koch drew an astonishing 75 percent of the vote, underscoring his great popularity with ethnic, blue collar, and Jewish voters. Despite his allegedly bad relations with Black leaders, a New York Daily News poll showed Koch pulled a sizable 37 percent of the Black vote in the 1985 Democratic primary compared to a Black candidate who polled only 40 percent, with 20 percent going to Council President Carol Bellamy.

"No New York mayor since Fiorello LaGuardia has inspired more affection, respect and outright loathing" asserted Insight magazine in 1985. Feisty, combative, spirited, and outrageously outspoken (he once called minority welfare spokesmen "poverty pimps"), Koch was also a fervent supporter of Israel and readily mixed municipal politics and foreign policy in his public addresses. During his second term Koch also wrote a best-selling book, Mayor: An Autobiography, an entertaining, shot-from-the-lip odyssey of mayoring which embarrassed his enemies and some of his erstwhile friends. Yet his larger achievement was that he brought New York back from the brink of financial disaster and raised the spirits of New Yorkers. For that and other reasons, Professor Roger Starr said of Koch: "He is the right man for the right time." Mayor Koch was New York City's most effective promoter and frequently repeated that he wanted to be "Mayor for Life."

Koch most recently tried his hand as a novelist, with Murder At City Hall (1995), and Murder on Broadway (1996). He also presided over a television revival of The People's Court that began in the fall of 1997.