[3] After receiving a degree in communications from AUT, Garner began his career at TVNZ in the mid 1990s, as a political reporter for veteran broadcaster Paul Holmes.
[4] He was praised in his early career for his scoop-heavy journalism, winning the 2004 Newspaper Publishers' Association award for Television Political News Reporter after exposing a $195,000 golden handshake received by then-Labour Party MP John Tamihere.
In 2017, Garner wrote in his Stuff.co.nz newspaper column of a "human snake" in a Kmart queue, to comment on high levels of South Asians and Syrians immigrating to New Zealand.
[13] Garner went onto present a mid-morning show for the news and talk radio station Today FM, which ceased broadcasting after just over a year on air.
He broke the story of a NZ$195,000 golden handshake that Labour Party MP John Tamihere received leaving the Waipareira Trust in October 2004.
[17][18] Tamihere was later cleared of financial impropriety, and the story won Garner the 2004 Newspaper Publishers' Association award for Television News Reporter – Politics.
[20] The tapes caught deputy leader Bill English saying he would sell Kiwibank, and MP Lockwood Smith saying National had to adopt policies the party did not like in order not to scare voters.
[5] In the wake of the 2010 MPs expenses scandal, Labour Party politician Chris Carter alleged that Garner had a vendetta against him.
The head of TV3 news and current affairs Mark Jennings replied saying Garner did not have a grudge, and "Carter is a serial offender on expenses ... it's Duncan's job to shine the light on that.
[7] In April 2018, Garner angrily responded to Taika Waititi's blunt description of racism in New Zealand in Dazed and Confused.
[30] Garner was a noted critic of Jacinda Ardern's premiership, and has claimed her "big promises have failed spectacularly".
Just over a year later, on 30 March 2023, Today FM ceased broadcasting, shortly after breakfast presenter Tova O'Brien interrupted Garner's show to announce the station's closure.