Duncan Garner started his career as a reporter for Television New Zealand in 1995 and worked on the current affairs show Holmes from 2000 to 2002. In 2003 he moved to TV3. He broke the story of the NZ$195,000 golden handshake Labour MP John Tamihere received leaving the Waipareira Trust in October 2004. The story prompted an investigation by the Serious Fraud Office, which caused Tamihere to resign from the Cabinet. Tamihere was later cleared of financial impropriety, and the story won Garner the 2004 Qantas Media Award for Television News Reporter - Politics.
In August 2008, three months before the New Zealand general election, Duncan 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 him a nomination for Best News Reporting in the 2009 Qantas Film and Television Awards.
In 2013, Duncan hosted two new current affairs shows on TV3 – 3rd Degree with Linda Clark and Guyon Espiner and The Vote. He co--hosted Story with Heather du Plessis-Allen from August 2015 to December 2016 before changing from nighttime to breakfast television beginning hosting morning news and talk show The AM Show in February 2017. It airs on Three and is simulcast on Radio Live. Duncan co-hosts with news anchor Amanda Gillies and sports presenter Mark Richardson.