King was born in New York, New York and grew up in Sunnyside, Queens. He is of Irish descent, with roots in Galway and Limerick. He graduated from St. Francis College. King served in the National Guard 1968 to 1973. King served as comptroller of Nassau County, New York from 1981 to 1993. King was elected to the House of Representatives in 1992. The 3rd congressional district is made up of middle class communities in eastern Nassau County, such as Hicksville, Levittown, Massapequa, Massapequa Park, Farmingdale, Seaford, and Wantagh, as well as some South Shore Suffolk County towns like Amityville, Copiague, Lindenhurst, West Babylon, Babylon, and West Islip. The district also includes the cities of Long Beach and Glen Cove.

Throughout his career, King has been one of the strongest supporters of the Irish Republican Army. During negotiations in 1998, King carried messages between the IRA and the British government. King voted against the impeachment of Bill Clinton and supported John McCain's candidacy in 2000. King has thought about running for the United States Senate both in 2000 and 2004 (against Hillary Clinton and Chuck Schumer respectively) but decided against it both times. He was also mentioned by fellow conservatives as possible candidates for Governor of New York and county executive of Nassau County.

King is the only Republican congressman left on Long Island, where Republicans once were the majority party. He has been reelected numerous times by large margins despite being a conservative Republican in a district that is increasingly moderate to liberal. His conservatism is tempered by a blue-collar appeal, which resonates with many of the ethnic Irish and Italian voters of his district.

King, an avid amateur boxer, is known for a very blunt attitude; in 2005 he generated controversy due to his heated replies to some letters from his constituents. In 1998 he was one of the few Republicans who voted against the Impeachment of Bill Clinton. In June 2005 he was quoted as saying that he supported President George W. Bush's position that there was a connection between Saddam Hussein and al-Qaeda.

In 2005 he stated that Valerie Plame deserved to be outed as a covert CIA operative. King also suggested that the media "be shot" for pursuing the story and identifying White House aide Karl Rove as the alleged leaker.

In February 2006, he made headlines for his strong disapproval of the Dubai Port Deal, pitting himself against Bush on this issue.

King is currently the Chairman of the United States House Committee on Homeland Security. A devout Catholic, he is also a House representative of the anti-abortion group Right to Life.

He resides in Seaford with his wife. Rosemary. He has two adult children and one grandchild.