Born on 16 April 1953, in Wahroonga, Sydney, Garrett attended Gordon West Public School and then Barker College in Hornsby before studying politics at the Australian National University (ANU), where he was a resident at Burgmann College, and later law at the University of New South Wales. His father died while he was still at school. His mother died in a fire at the family home, during his period at university.

In 1973, Garrett became the lead singer for the rock band Midnight Oil, after responding to an advertisement placed by one of the band’s founding members, Rob Hirst. In tandem with their musical and commercial success, the band were strongly identified with environmental and Aboriginal rights causes. They were particularly critical of the military and foreign policy of the United States during the 1980s. On and off stage, Midnight Oil often made political statements. At the closing ceremony of the 2000 Olympic Games in Sydney, the group performed before then-Prime Minister, John Howard, and a television audience of hundreds of millions, wearing black overalls bearing the word “sorry”. This referred to the Howard Government’s refusal to apologise to Aboriginal Australians for the former policy of removing Aboriginal children from their families.

Garrett’s first attempt to enter the Australian Parliament came in December 1984, when the Nuclear Disarmament Party (NDP) invited him to stand for a New South Wales seat in the Australian Senate at the federal election. He refused at first, but after consulting his bandmates, he agreed on the condition that he head the ticket. He needed 12.5% of the statewide vote to win a seat in the Senate voting system, but a primary vote of 9.6% proved insufficient when Labor allocated its second preferences to the Democrats ahead of the NDP.

During his time with Midnight Oil, Garrett served two terms as President of the Australian Conservation Foundation (from 1989 to 1993 and 1998 to 2004. He was also invited to join the international board of Greenpeace in 1993 for a two-year term. He served as an adviser and patron to various cultural and community organisations, including Jubilee Debt Relief, and was a founding member of the Surfrider Foundation. In 2000, Garrett was awarded the Australian Humanitarian Foundation Award in the environment category and in 2001 he received an honorary Doctorate of Letters from the University of New South Wales. Garrett announced his departure from Midnight Oil in 2002, saying he wished to concentrate more fully on his environmental and social activism.

Although no longer part of the band, Garrett performed several times with Midnight Oil in the following years. After the Asian tsunami on 26 December 2004, Garrett joined the other members of Midnight Oil to perform as part of the fund-raising event WaveAid. On 7 July 2007, Garrett presented Crowded House at the Australian leg of Live Earth. On 14 March 2009, Garrett performed live at the Melbourne Cricket Ground with Midnight Oil for Sound Relief, in order to raise money for the Victorian bushfire appeal. In November 2012, Garrett and Paul Kelly inducted Yothu Yindi in the ARIA Hall of Fame and performed with the band their best-known song “Treaty”.

After the conclusion of Garrett’s term as President of the Australian Conservation Foundation ended, in June 2004 Labor Leader Mark Latham announced that Garrett would become an Australian Labor Party candidate for the House of Representatives at that year’s federal election, in the safe New South Wales seat of Kingsford Smith which was being vacated by the former Cabinet Minister Laurie Brereton. There was some initial criticism from Labor members within the electorate, as the local branch had wished to select their own candidate; in the end, Garrett was comfortably elected for the seat in October 2004. Six months after entering Parliament, Garrett was appointed Shadow Parliamentary Secretary for Reconciliation and the Arts.

Garrett campaigned for Labor in the 2006 Victorian state election, causing some controversy when he sent a letter to voters in the seat of Melbourne, where Labor and the Greens were embroiled in a tight contest. In the letter Garrett urged voters not to vote for the Greens, claiming they were in alliance with the conservative Liberal Party. This incurred the ire of Greens Leader, and former Garrett ally, Bob Brown, who accused Garrett of having “sold out” and of going against the environmental movement since joining the Labor Party.

In December 2006, Kevin Rudd appointed Garrett to his Shadow Cabinet as Shadow Minister for Climate Change, the Environment, Heritage and the Arts. Garrett supported Rudd in that month’s leadership spill, a decision he would later come to regret, saying years later that it was “certainly the biggest mistake” he made in his political career.

During the 2007 election campaign, Garrett caused some controversy after the journalist Steve Price claimed that Garrett had said to him Labor would change their policies if put in power. This was disputed by Garrett as a “short jocular conversation”. Garrett was comfortably re-elected for Kingsford Smith in the election, with a 4.56% swing towards him.

After Labor’s victory in the 2007 election, Prime Minister Kevin Rudd appointed Garrett as Minister for the Environment, Heritage and the Arts in the Cabinet, although responsibility for climate change was separated from the portfolio and handed to Senator Penny Wong. Although Garrett requested to represent the Government on climate change within the House of Representatives, Rudd instead gave that responsibility to Treasurer Wayne Swan.

In 2007 the artist Michael Mucci entered a portrait of Garrett in the Archibald Prize.

In his first acts in the role, Garrett approved a controversial plan to dredge Melbourne’s Port Phillip Bay. This move attracted strong criticism from some environmental groups, who were concerned that the 23 million cubic metres of sand, rock and contaminated silt dredged from the bay’s shipping channels would affect fishing and tourism in the area. Garrett also refused federal funding that would have enabled a remount of Elke Neidhardt’s acclaimed Adelaide production of Der Ring des Nibelungen in 2008.

In August 2008, Garrett approved a major expansion of South Australia’s Beverley uranium mine, saying the mine would demonstrate global best practice for environmental protection. Garrett’s decision was praised by the uranium industry, but criticised by the Australian Conservation Foundation, the organisation Garrett previously led, which said the decision would result in the mine spreading acid and radioactive pollution over 100 square kilometres (39 sq mi). Later that year, Garrett announced the withdrawing of all A$2.6 million funding from the Australian National Academy of Music (ANAM).

In 2009, Garrett chose not to give the proposed A$2 billion Tamar Valley mill approval until more studies were undertaken on its potential impact on marine environments. A new condition was put on the mill, meaning Gunns could be liable for criminal and civil penalties if the mill was approved and broke defined “environmental limits”. That same year saw Garrett also rejected proposals to impound the Mary River through the construction of the Traveston Crossing Dam. Garrett determined that the impacts of the proposed dam on the threatened species of Australian lungfish, the Mary River Turtle and the Mary River cod would be too great and of national environmental significance.

In a 2010 reshuffle of his Cabinet, Rudd demoted Garrett to the outer ministry in response to Garrett’s administration of the Home Insulation Program (HIP), which was linked to four deaths, over 100 house fires and allegations of fraud. It was subsequently revealed in May 2010 that Garrett had written to Rudd on four occasions raising concerns about safety. Following the leadership spill in June 2010, which saw Rudd replaced by Julia Gillard, Garrett’s colleague Gary Gray revealed most MPs believed Garrett was demoted because “…Rudd had a shocking interview on The 7.30 Report and needed a scapegoat”. Gray stated:

In September 2011, Garrett announced an alteration of the National School Chaplaincy Program by offering schools the opportunity to employ, instead of “a religious support worker” (chaplain), a “secular student well-being officer”. In February 2012, Garrett, alongside Indigenous Affairs Minister Jenny Macklin, announced an expansion of the Improving School Enrollment and Attendance through Welfare Reform Measure (SEAM), a program through which parents of indigenous students in the Northern Territory can have their Centrelink payments suspended for three months if their children are not attending or enrolled in school. Though the Government claimed that the program improved indigenous school attendance, it was heavily criticised by some indigenous spokespeople and academics for being excessively punitive rather than implementing other policies that may have been far more effective in improving school attendance.

In 2013, Garrett pledged to increase funding to public schools as recommended in the Gonski Report, in order to reduce inequality in educational performance. In a 2014 book review, Garrett stated that he was concerned at the growth of private schools, which could lead to “an increasingly segregated school system”. He noted that the National School Chaplaincy Program needed to change their guidelines because “the line between chaplains acting to support students in the provision of general pastoral care and proselytising was too easily crossed”.

Garrett supported Gillard in both 2013 leadership spills. After the June spill, where Rudd defeated Gillard, Garrett immediately announced his resignation as Education Minister, and the following day declared that he would not seek re-election at the 2013 election, ending his career in Parliament after nine years.

On 5 May 2016, after the conclusion of his career in Parliament, Garrett announced that Midnight Oil would be reforming and that they would be touring in 2017, including a trip to the United States. Midnight Oil stated on their Facebook page: “We wanted you to be the first to know that the five of us are planning to do some gigs in Australia and overseas during 2017.” Days later, Garrett also announced his first solo album, A Version of Now, which was released on 15 July.