Carl Katter was born in Brisbane, Queensland, and he grew up in the northwest of the state. His parents were federal MP Bob Katter, Sr. and Joy Katter, and he is a member of a widely respected pioneering Queensland family. Carl came out as gay at the age of 18. He grew up gay in Far North Queensland and experienced many of the negative attitudes of those in his community towards lesbian, gay, bisexual, and transgender (LGBT) people, including once being beaten up in Charters Towers, the town where he grew up, because of his sexuality. This left an indelible mark on Carl's character, and contributed to him becoming an activist. In an interview, Carl once described his fear as a same-sex attracted young person in Northern Queensland:
"Having travelled the world as an adult and walked the streets of many international cities, I have never felt as scared as when I would walk the streets of Charters Towers after dark...I am definitely one of the lucky ones; I got out and avoided having to resort to taking my own life during those hard years."
Bob Katter Sr, Carl's father, who died when Carl was 12 years old, was a member of the Australian Parliament representing the federal electorate of Kennedy. The younger Katter was heavily influenced by his father's labour and trade unionist politics, and by his father's belief in equality for people of all races.
In August 2011, Bob Katter Jr, Katter's half-brother and member of Parliament representing the seat of Kennedy, like his father before him, appeared at a right wing Christian rally at the Great Hall in Parliament House and decried same-sex marriage. Carl reportedly watched on television as his half-brother mocked same-sex marriage and suggested that it "deserves to be laughed at and ridiculed."
Deciding publicly to come out regarding his own homosexuality, Carl Katter join the push for the legalisation of same-sex marriage in Australia. He approached GetUp!, an independent Australian grass-roots community advocacy organisation, and with their help produced an online video message to counter his brother's comments. In a television interview with George Negus, Katter said that he could not "just sit back" and allow Bob Katter Jr. to denigrate gay people and the campaign for same-sex marriage, saying: "it's hurtful, it's dangerous, and it's really inappropriate."
Katter has stated that he believes that it is better to combat his brother's views and educate rather than attack his brother on a personal level. He also holds the work that his brother has done for the Kennedy electorate and elsewhere in high regard.
Carl is a member of the Labor Environment Action Network, and wants to continue to build on Labor’s world-leading approach to climate change in the implementation of the party's policy when it is next elected to national government.
In 2011, Katter was included in The Age's Top 100 most influential Melburnians.
He was a guest speaker at the Metropolitan Community Church's 2011 annual Christmas event, hosted by Justice Michael Kirby, where he gave a speech about marriage equality.
In 2014, Katter was asked to be the keynote speaker at the Australian Intervarsity Debating Championships, also known as the Easters, hosted that year by Monash University.
He was the Labor candidate for Higgins for the 2016 Federal Election. He won 15% of the vote.