Percy was born on 28 September 1943 in Tolpuddle, England, and trained as a motor mechanic. He entered his first race in 1964 driving his own road-going Ford Anglia 1200 in a local time-trial, winning despite competing against far more powerful cars. While he initially pursued racing as a hobby, his natural talent produced consistent high-placed finishes at national level. A notable early success came in 1973 when he won all three televised rallycross races at Cadwell Park. He turned professional in 1974, driving Spike Anderson's Samuri Datsun 240Z in the British Modified Sports Car Championship — and won that too.
Percy entered the British Touring Car Championship for the first time in 1975, driving a Toyota. His first BTCC race also marked his first encounter with Tom Walkinshaw, whom Percy challenged by attacking the class above his own. He remained loyal to Toyota for several seasons before Walkinshaw persuaded him to join Tom Walkinshaw Racing for the 1980 season in a Mazda RX-7. Percy won the 1980 BTCC title for TWR and repeated the feat in 1981. He then returned to Toyota for 1982 and claimed a third BTCC crown in a Toyota Corolla. In 1983, Percy also won the Willhire 24 Hours in a Porsche 928S, demonstrating his versatility across different machinery.
Despite competing in the BTCC with Toyota in 1983, Percy maintained close ties with TWR through occasional drives in the Jaguar XJS, which was dominant in Group A racing at the time. Walkinshaw brought him back full-time in 1984 to campaign Jaguars in the European Touring Car Championship. The TWR squad won the 1984 ETCC team title while Walkinshaw took the drivers' championship; the car shared by Walkinshaw, Percy, and Hans Heyer won the Spa 24 Hours, the ETCC's most prestigious round.
In 1985, with Jaguar shifting focus to sports car racing, TWR switched to factory-backed Rover Vitesse V8s for the ETCC. Percy and Walkinshaw finished third in the drivers' championship that year while recording victories in seven 500 km rounds including Donington, Silverstone, Monza, and the Österreichring. The 1986 ETCC season produced perhaps Percy's most bittersweet result: he was initially declared champion before the FIA retroactively applied a dropped-scores rule a month after the season's end, handing the title to Roberto Ravaglia of Schnitzer Motorsport. Percy finished second in the final standings.
During this period Percy also competed at Le Mans. In 1986 he co-drove a TWR Jaguar XJR-6 Group C car with Gianfranco Brancatelli and Hurley Haywood. His 1987 Le Mans entry ended dramatically when a tyre blowout at approximately 240 mph on the Mulsanne Straight tore off the rear bodywork and flipped the car. The wreckage came to rest 600 metres down the road, yet Percy walked away with nothing worse than a battered helmet. He also contested the 1989 Spa 24 Hours, winning in an Eggenberger Motorsport Ford Sierra RS500.
Percy's connections with Walkinshaw brought him to Australia for the Australasian rounds of the 1987 World Touring Car Championship, where he co-drove with Allan Grice in a Roadways Racing Holden Commodore VL. He returned for the 1988 Bathurst 1000 and again raced with Perkins Engineering in 1989 under the Holden Racing Team banner.
In 1990, Percy established the Holden Racing Team for Tom Walkinshaw, taking on dual roles as team manager and lead driver for the Australian Touring Car Championship. The year culminated in a surprise victory at the Bathurst 1000 alongside Grice in a Holden Commodore VL. They followed that with second place at Bathurst in 1991. After stepping back at the end of 1991, Percy returned to Australia and the HRT in 1992, finishing fifth outright and first in Class C at Bathurst alongside Grice once more.
He continued to contest Australian endurance events through the 1990s with Wayne Gardner Racing and John Faulkner Racing, while also managing the Mazda BTCC entry in 1994 and serving as chief tester and team manager for Harrier between 1995 and 1997. As a driver during that period, he scored the Jaguar XJ220's first race win.
From the late 1990s Percy became active in historic motorsport, frequently campaigning his Jaguar D-type. His performances in this arena were outstanding; at the 2002 Le Mans Classic meeting he won all four races he entered.
Percy's career was ultimately curtailed by a serious accident in his garden in summer 2003, after which a medical error during his hospital treatment left him paralysed from the waist down. He pursued a successful legal case against the West Dorset General Hospital NHS Trust, receiving an out-of-court settlement of £1.55 million in April 2008. In 2013 he suffered a heart attack. Despite no longer being able to compete, Percy has remained a familiar presence at racing events throughout Britain.
His career stands as one of the most varied and decorated in British motorsport: three BTCC titles, a European Touring Car Championship, the Spa 24 Hours twice (1984 and 1989), the Bathurst 1000, and a record that took him from grass-roots rallycross to Group C Le Mans machinery and back again.