King was born in Murchison, a town in the Tasman region of the South Island. After receiving primary education in Murchison, she attended Murchison District High School and Waimea College. She gained a BA degree from the University of Waikato. She then obtained a post-graduate diploma in dental nursing, and worked as a dental nurse from 1967 to 1981. She was a tutor of dental nursing in Wellington from 1982 to 1984. She is partly of Sri Lankan descent.
King joined the Labour Party in 1972, and has held various offices within the party, including a term on the party's executive (1991–1992).
In the 1984 elections, she stood as the party's candidate for Horowhenua, and was successful. She was re-elected in the 1987 election.
Following the 1987 election, she was appointed parliamentary under-secretary to the Minister of Employment and of Social Welfare. In 1989, she was elevated to Cabinet, becoming Minister of Employment, Minister of Immigration, and Minister of Youth Affairs. She was also given special responsibility for liaising between Cabinet and the party caucus.
In the 1990 election, King lost the Horowhenua electorate against Hamish Hancock, a lawyer who stood for the National Party. She served as chief executive officer of the Palmerston North Enterprise Board from 1991 until the 1993 election, when she was returned to Parliament as the MP for Miramar. In the 1996 election, when the shift to mixed-member proportional (MMP) representation prompted a reorganisation of electorates, King successfully contested the new seat of Rongotai, which she still represents. In that 1996 election, she was ranked in sixth place on the Labour Party's list.
When Labour won the 1999 election, and Helen Clark became Prime Minister, King was appointed Minister of Health. She was ranked sixth within Cabinet. After Labour winning a third term in government at the 2005 election, King took on the roles of Minister of Transport and Minister of Police. Following another reshuffle in late 2007, King became the new Minister of Justice. Before the 2008 general election she was elevated to number four on the party list.
Labour was defeated in the 2008 election by the National Party led by relative newcomer John Key. King retained her seat with a majority of about 7,800 votes. King became the deputy leader of the Labour Party in a special caucus meeting on 11 November 2008 replacing former Deputy Leader and Deputy prime minister Michael Cullen. Phil Goff, another senior Labour Party member, became the leader of the Labour Party, replacing former Prime Minister Helen Clark. King stood again for Rongotai in the 2011 general election. She was ranked second on the Labour Party list. Following the defeat of the Labour Party in the 2011 election, Annette King announced she would step down as Deputy Leader of the Labour party, and Deputy Leader of the Opposition effective 13 December 2011. She was succeeded as Deputy Leader by Grant Robertson in the 2011 Labour Party leadership election.
In the 2014 election, King increased her majority in the Rongotai electorate, but National won the party vote for the first time since the initial MMP election in 1996. Labour's heavy defeat at the 2014 election caused the resignation of David Cunliffe as the party's leader and the next leadership election, with King in an interim capacity as deputy leader.
Following the election of Andrew Little as the new leader, King remained as deputy in a permanent capacity although Little has guaranteed that she will be deputy for at least a year and did not indicate whether he wants her to be a future Deputy Prime Minister.
On 1 March 2017 King announced her intention to retire from politics at the 2017 election, despite initially indicating she would only contest the election on the party list. She has also stepped down from the deputy leader role.
In 2007, King was awarded a Bravo award by the New Zealand Skeptics for her work along with "industry group Natural Products New Zealand, their attempt to provide standards and accountability via the Therapeutic Products and Medicines Bill."
King is married with one daughter, and has three step-sons. She is a cousin of National minister Chris Finlayson, from whom she received verbal abuse in Parliament in September 2013. Finlayson has also opposed her in the Rongotai electorate since the 2008 election.