At Murdoch University, Bandt was a student activist and member of the Left Alliance. During university, Bandt stated he was inspired by the works and thought of Leon Trotsky. He was president of the student union and an active campaigner for higher living allowances for students, and for free education. While he was a student in 1995, Bandt described the Greens as a “bourgeois” party, but that supporting them might be the most effective strategy, saying that “Communists can’t fetishise alternative political parties, but should always make some kind of materially based assessment about the effectiveness of any given strategy come election time”.

Bandt was born in Adelaide, South Australia—a descendant of German immigrants who emigrated to the Hahndorf and Barossa Valley regions in the 1800s. When he was a child, his family moved to Perth, Western Australia where he attended high school and Murdoch University. He graduated in 1996 with Bachelor of Arts and Bachelor of Laws degrees, and was awarded the Sir Ronald Wilson Prize for Academic Achievement, “which is given to the graduate who best combines distinguished academic performance in law units with qualities of character, leadership and all-round contribution to the life of the university”.

Following the 2007 federal election Melbourne had become Australia’s only Labor/Greens marginal seat. Bandt was preselected as Greens candidate for the second time, and ran successfully against a new Labor candidate, Cath Bowtell, following Lindsay Tanner’s retirement. Bandt received a primary vote of 36.2 percent and a two-party-preferred vote of 56 percent against Labor, a swing to him of 13.4 and 10.8 points, respectively. He was elected on the ninth count after over three-quarters of Liberal preferences flowed to him, enabling him to overtake Bowtell and become the first Green candidate to win a seat in a general election.

In 2008, Bandt completed a PhD at Monash University, supervised by cultural theorist Andrew Milner, with his thesis titled “Work to Rule: Rethinking Marx, Pashukanis and Law”. In 2012, he described his thesis as looking “at the connection between globalisation and the trend of governments to take away peoples’ rights by suspending the rule of law”, saying he “reviewed authors who write about the connection between the economy and the law from across the political spectrum”, ultimately arguing “that governments increasingly don’t accept that people have inalienable rights”. He has said that he had the thesis suppressed for three years in the hopes of having it published as a book.

In 2009, Bandt published a paper analysing how emergencies, such as the global financial crisis and war on terror, have been used by neoliberal “strong states” to “undermine basic rights”.

Prior to his election to parliament in 2010, Bandt lived in Parkville, Victoria and worked as an industrial and public interest lawyer, becoming a partner at Slater & Gordon. He had articles published on links between anti-terror legislation and labour laws and worked on issues facing outworkers in the textiles industry. Bandt says he also represented firefighters and coal workers confronting the threat of privatisation.

In 2013 Bandt was re-elected to the seat of Melbourne, despite an overall decrease in the Greens’ vote and Liberal Party directing preferences to Labor ahead of The Greens. Bandt retained the seat with a 42.6 percent primary and 55.2 percent two-party-preferred vote, with his two-candidate majority almost untouched. Bandt sat on Christine Milne’s frontbench.

In 2015, upon the change of Green leadership from Christine Milne to Richard Di Natale, Bandt did not re-contest the deputy leadership saying he had a baby due in the upcoming weeks. Scott Ludlam and Larissa Waters were elected unopposed as co-deputies.

Bandt was re-elected as Member for Melbourne in the 2016 election for a third time, pushing Labor into third place, and the overwhelming preference for him over the Liberals from Labor voters allowed him to increase his two-candidate-preferred vote to 68.48%. In 2017, the Party’s co-deputy leaders Larissa Waters and Scott Ludlam were found to be ineligible to sit in Australia’s Parliament owing to their status as dual citizens. Rachel Siewert and Bandt were made temporary co-deputy leaders. Bandt achieved national headlines in February 2018 for accusing new senator Jim Molan of war crimes after it was revealed that Molan had shared anti-Muslim content made by far-right party Britain First on their Facebook account. Bandt later apologised.

On 3 February 2020, Richard Di Natale announced his resignation as leader of the Greens and imminent retirement from politics, citing family reasons. Bandt announced his candidacy for the leadership shortly after. On 4 February, he was elected unopposed. Larissa Waters was elected unopposed as co-deputy, with Nick McKim defeating Sarah Hanson-Young and Mehreen Faruqi to become the second co-deputy. Bandt has been described by the political journalist Paddy Manning as the first Greens leader from the Left wing of the party.

Bandt has been described as different to previous Greens leaders due to his emphasis on “public ownership, public wealth, and community-driven responses to the links between climate change and capitalism.” Following Virgin Airlines Australia undergoing voluntary administration in 2020, Bandt called for the government to purchase the airline “at bargain basement prices”.