Paffett began karting in 1993 and showed immediate talent, winning the British Junior TKM Championship in 1995. He dominated single-seater categories from an early age: in 1998 he won the Formula Vauxhall Junior championship with thirteen pole positions and thirteen wins, setting track records across the season. He was awarded the prestigious McLaren Autosport BRDC Young Driver of the Year award at the end of that season.
Moving to British Formula Three, Paffett won the Scholarship class championship with twelve wins, thirteen fastest laps, and thirteen pole positions. He then went to the German Formula 3 Championship, driving for Keke Rosberg's team, and won that title in 2002 after dominating the field.
Paffett's route into the DTM came after a planned 2003 F3000 campaign with Brand Motorsport collapsed when the team withdrew from the series. Mercedes contacted him and signed him to race in the DTM with Rosberg's Mercedes AMG squad. In his second season, 2004, he finished runner-up with four wins. He won the championship in 2005 with five wins and four pole positions, establishing himself as a leading force in the series.
After seasons in older specification machinery, Paffett returned to a current Mercedes for 2009 and again finished runner-up to Timo Scheider. At Oschersleben in 2007, while in a year-old car with Persson Motorsport, he became the first driver in DTM history to win in older machinery.
In 2018, Mercedes announced it would leave DTM at the end of that season. Paffett responded with one of the best campaigns of his career, winning at Hockenheim, Zandvoort, and three other rounds, collecting three victories, three second-place finishes, and four thirds to take the championship title in the team's farewell season.
Despite his speed and consistency, Paffett never secured a permanent Formula One race seat. He served as a full-time test driver for McLaren-Mercedes from 2006, having been linked with a 2007 race seat when Kimi Räikkönen departed for Ferrari — a seat that went to GP2 champion Lewis Hamilton instead. He was released from his McLaren contract in late 2006 but re-signed as the team's second test driver shortly afterwards.
Paffett acted as Force India's reserve driver at the 2012 Australian Grand Prix and drove simulator duties for Williams from 2016. His McLaren test role continued until the end of 2014, when the team switched to Honda engines.
In the 2018–19 Formula E season, Paffett drove for the HWA Racelab team alongside Stoffel Vandoorne. It was his debut in the all-electric series and HWA's first campaign. He scored points in Hong Kong, Paris, and New York City, finishing nineteenth in the standings. In September 2019, he was replaced by Nyck de Vries in the incoming Mercedes-Benz EQ works team and moved into a sporting and technical advisory role within that organisation, also serving as a test and reserve driver.
Paffett's two DTM titles — achieved in 2005 and 2018, separated by thirteen years — and his record-breaking win in a year-old car at Oschersleben mark him as one of the most capable touring car drivers of his generation. His long association with Mercedes across DTM, Formula One testing, and Formula E reflects a career built on technical precision and consistency rather than high-profile race seats.