Duncan Garner started his career as a reporter for Television New Zealand in 1995, working on the current affairs show Holmes from 2000 to 2002.
Garner then moved to rival TV3 in 2003.
In August 2008, three months before the New Zealand general election, Garner broadcast audio tapes secretly recorded at a National Party cocktail party by Kees Keizer.The tapes caught deputy leader Bill English saying he would sell Kiwibank, and future Speaker of the House of Representatives Lockwood Smith saying National had to adopt policies the party did not like in order not to scare voters.The broadcast earned Garner a nomination for Best News Reporting in the 2009 Qantas Film and Television Awards.
In March 2010, TV3 launched the weekend current affairs program The Nation, with Garner as co-host.
In 2013, Garner hosted two new current affairs shows on TV3 – 3rd Degree and The Vote.