Bartlett was born in Brisbane, where he has lived all his life. He is of Irish, Swiss, English and Greek origins – his great-great-grandfather, who is claimed to be the first Greek settler in Australia, arrived in Adelaide in 1840. He was educated at the University of Queensland in the 1980s, where he graduated in arts and social work. Before entering politics, Bartlett was a social worker with the Department of Social Security, and worked with alternative radio station, 4ZZZ FM, in roles including announcer and finance coordinator. He played in a number of local rock bands around this time, as a drummer and keyboard player. The bands were I Am Vertical, The Cutters and Too Green For Summer (who years later had a song appear on the Rock Against Howard CD).
In 1990, Bartlett joined the staff of Queensland senator Cheryl Kernot. Three years later, he joined the staff of Democrats senator John Woodley as an adviser and researcher. He was the Democrats' Queensland campaign director for the 1993 and 1996 elections and federal campaign director in 1998. Bartlett was appointed in 1997 to the Senate casual vacancy caused by the resignation of Kernot, and was elected for a six-year term at the 2001 federal election.
A consistent and vocal campaigner for refugees and asylum seekers, Bartlett was the only Australian parliamentarian at that time to have visited every refugee detention centre in Australia, as well as those on Christmas Island and Nauru (detention centres off the Australian mainland, see Australia's Pacific Solution) where he went four times to meet with detainees.
Bartlett initiated the Senate Inquiry into Australia's refugee determination system which produced the "Sanctuary Under Review" report in 2000, and has participated in numerous other committee inquiries into immigration matters.
Bartlett has spoken many times on behalf of those living in poverty, as well as the physically and mentally disabled. He also takes a close interest in the environment and animal welfare. In 2003 he introduced a private member's bill to overhaul the animal welfare system in Australia. His petition to end the live sheep export trade received well over 100,000 signatures.
Bartlett was a strong opponent of Australia's involvement in the 2003 invasion of Iraq. He introduced a private member's bill designed to ensure no Prime Minister of Australia could again send the country to war without the consent of both houses of parliament. When the bill was debated in the Senate, speakers from both major parties indicated their opposition to it, although there was no formal vote taken.
Bartlett has also campaigned strongly for gay rights. In 2004, he cried in the Senate chamber over a proposed law to define marriage as between a man and a woman, which he called an "absolute disgrace".
Bartlett co-sponsored the Cluster Munitions (Prohibition) Bill 2006, which was introduced into the Senate on 5 December 2006. If enacted, it would prevent Australia from using, manufacturing or possessing cluster munitions.
After the resignation of then party Senate leader Natasha Stott Despoja on 21 August 2002, Bartlett was elected to the Democrats Party leadership in October, supplanting the temporary leader Brian Greig.
To a degree, Bartlett stabilised the Democrats' troubled party room and spoke strongly against the Government's maltreatment of refugees and maladministration of the Department of Immigration. He also oversaw the Democrat senators' use of their potential balance of power role to influence increased funding for Medicare, protection of the welfare payments of sole parents, the unemployed and the disabled, and entitlement of some homosexual couples to superannuation entitlements equivalent to those enjoyed by heterosexual couples.
In December 2003, Bartlett took leave from his Senate leadership after an incident involving Liberal Senator Jeannie Ferris when leaving the Senate chamber after a vote. Bartlett, who had been drinking at a Liberal Party function held just outside the chamber, was accused of stealing five bottles of wine from the function. Some time after Ferris had retrieved the wine, Bartlett approached Ferris, and was alleged to have gripped her arm and verbally abused her, both inside the chamber and along the way to an outside courtyard. Parliamentary video of part of the incident appeared to show that Bartlett was drunk in the chamber, although did not show him grabbing Ferris's arm. Bartlett's subsequent formal apology was accompanied by a bottle of wine, which Ferris described as "quite inappropriate ... as an apology for drunken behaviour involving abuse and a physical attack." By contrast, Liberal Senator Brett Mason, who witnessed the incident, said "Perhaps a little more was made of the incident than should have been made. I think it was overplayed by the media, and by everyone." Labor Senator Claire Moore was reported in The Bulletin magazine as saying Bartlett had been "unfairly demonized."
Bartlett resumed the party's parliamentary leadership in January 2004, giving an assurance that he would totally abstain from alcohol. However, the party's support levels remained at the same low level to which they had fallen at the time of Stott Despoja's resignation. He was unable to increase the party's support leading up to the 2004 election in which the Democrats were defending three Senate seats. All three seats were lost—one going to the Greens and two to Liberals. The party polled what was at the time the lowest vote since their inception in 1977.
Following the 2004 election, Bartlett did not re-contest the leadership, instead taking on the deputy leadership under Lyn Allison. Bartlett was defeated at the 2007 election, polling only 1.88% of the primary vote in Queensland. The Democrat vote was even lower in other states, and the party lost all its remaining Senate seats. He left the Senate at the expiration of his term in June 2008.
He took up a position as a part-time Research Fellow with the Migration Law Program at the Australian National University. He has since returned to being an announcer on Brisbane's 4ZZZFM radio station, and he occasionally writes pieces for websites such as Crikey, New Matilda, The Drum and Online Opinion.
In November 2009 Greens leader Bob Brown announced that Bartlett would contest the lower house seat of Brisbane at the 2010 federal election as a candidate for the Australian Greens. Bartlett came third in the seat in the 2010 election, gaining 21.3% of the vote with a swing to the Greens of just over 10%.
In May 2012, Bartlett ran for the Lord Mayoralty of Brisbane for the Greens, receiving 10.7% of the primary vote, a 2.3% increase on the previous election.
In 2015, years after the Democrats' parliamentary oblivion, the party was deregistered by the Australian Electoral Commission. Speaking as a former Democrats leader, Bartlett reflected that the party's support of the Howard Government's introduction of the GST was "politically catastrophic", but the "last straw" for the party was the demise of Stott Despoja as leader in 2002:
Even though the Democrats eventually disappeared from parliament in 2008, basically our political support crashed and burned in 2002.
Bartlett was endorsed by the Australian Greens as a Senate candidate for Queensland at the 2016 federal election. While he did not meet the quota for election, his colleague Senator Larissa Waters resigned her position on 18 July 2017 after discovering she held dual Australian and Canadian citizenship. As the second on the 2016 Australian Greens Senate ticket, he is expected to replace her after a recount.
Bartlett is active on animal rights and human rights issues.
On his personal blog, Bartlett has noted that he is a fan of both Joy Division and Nick Cave. Bartlett has also returned to playing in the local music scene, currently playing drums for a group called The Hinterlanders.
In Mental Health Week 2013, Bartlett wrote an article for the Courier-Mail about his being hospitalised for depression in 2012.
He has one daughter, Lillith.