Dumbrell grew up in Melbourne, educated at Xavier College in Kew, and began racing in karts in 1996. By 1998 he had attended the Jim Russell International School of Racing in England, where he drove a Formula Vauxhall and was the youngest graduate — ineligible for the final race at 15 because drivers needed to be 16. On returning to Australia he broke his hip in a training accident, ending plans for a Formula Ford campaign that year. He made his first Bathurst 1000 start aged 17 years and 74 days, making him at the time the youngest driver ever to pilot a V8 Supercar.
Dumbrell's early career featured a mix of junior formulae and Supercars co-driver roles. He won the Konica V8 Supercar Development Series in 2002 before securing a full-time drive with Castrol Perkins Motorsport for 2003 and 2004, having impressed with a fifth-place finish at Bathurst 2002 alongside Larry Perkins.
His best championship standing came in 2006, finishing twelfth while nearly threatening a podium at Bathurst before mechanical problems intervened. In 2007 he moved to Supercheap Auto Racing, then joined the HSV Dealer Team in 2008 as teammate to 2006 champion Rick Kelly. He moved to Walkinshaw Racing in 2009 before signing with Rod Nash Racing for 2010–2012, marking his first experience racing a Ford.
With Rod Nash Racing, Dumbrell achieved his first V8 Supercar Championship podium finish in 2010 at the Falken Tasmania Challenge, followed immediately by his first race win and first pole position at the Norton 360 Sandown Challenge. He announced retirement from full-time racing in August 2011 at season's end, citing increasing business commitments.
Retirement from full-time racing did not end Dumbrell's involvement. From 2012 he joined Triple Eight Race Engineering as co-driver to seven-time series champion Jamie Whincup in the two-driver endurance events. The partnership was immediately successful: they won the Bathurst 1000 in 2012. In 2013 they won the Sandown 500, and in 2014 they won the Sandown 500 a second time, taking the Enduro Cup as the highest-scoring driver combination across the endurance events. Dumbrell also won the Dunlop Series development championship for a second time in 2014.
Dumbrell balanced motorsport and commerce throughout his career, owning a Boost Juice franchise as early as 2002 and co-founding a media company. He progressed through the family business, the Automotive Brands Group — owners of Autobarn and Autopro — becoming chief executive officer in 2009. Automotive Brands was sold to Metcash in 2012 with Dumbrell remaining in an executive role. In 2015, the business was sold again, to Burson Group, in an AU$275 million deal, with Dumbrell staying in charge of the automotive division.
Dumbrell's Bathurst 1000 victory in 2012 with Jamie Whincup at Triple Eight confirmed that his post-full-time career in endurance racing could deliver results at the highest level. His dual success as a race winner and major corporate executive made him an unusual figure in Australian motorsport.