King was born in Dorchester, Massachusetts. He attended Boston Latin School, and earned a bachelor's degree in journalism from the University of Rhode Island. On May 25, 2008, King married fellow CNN reporter Dana Bash. Before marrying Bash, King converted to Judaism, Bash's religion. He is not related to fellow CNN personality Larry King.

In 1985, King joined the Associated Press where he began as a writer. In 1991, King was named chief political correspondent and headed the AP's political coverage of the 1992 and 1996 presidential elections. Also in 1991, King won the top reporting prize from the Associated Press Managing Editors' Association for his coverage of the Gulf War in Kuwait.

In 1997, King joined CNN where he served as the senior White House correspondent from 1999 to 2005. In 2005, King was named CNN's chief national correspondent, a position he still holds. He frequently appears on the nightly news programs The Situation Room and Anderson Cooper 360 and sometimes fills in as anchor.

King also uses the Multi-Touch Collaboration Wall, nicknamed the "Magic Wall" or "Magic Map." First used during the primaries of the 2008 presidential campaign, it allows him to display and manipulate various graphics and maps relating to poll and election results. He and the Multi-Touch Collaboration Wall have both been featured in a Daily Show segment.

Just prior to the 2009 US presidential inauguration, King began hosting his new talk show State of the Union, which replaced CNN's Late Edition with Wolf Blitzer. It was announced on November 12, 2009 that King will host a show in the timeslot formerly occupied by Lou Dobbs in early 2010. On January 31, 2010, King announced that CNN's senior political reporter, Candy Crowley, would replace him as anchor of State of the Union. King began hosting a new show on CNN starting March 22 called John King, USA.