Tim Parnell grew up accompanying his father to race meetings across Britain and Europe from childhood, attending the 1938 Donington Grand Prix as a small boy. His father discouraged active involvement in the sport due to its dangers, but Parnell eventually purchased a Frazer-Nash Le Mans Replica and entered his first race at Mallory Park in 1957. He then raced a Formula 2 Cooper-Climax T45 for two seasons, becoming a regular in the national F2 circuit travelling around Britain and Europe. He won races at Mallory Park in June 1959 against a strong field and at Silverstone, and finished third at the F2 race at Whitchurch airfield near Bristol.
For 1960 he acquired a pair of Lotus Type 18s for F2 and Formula Junior alongside the aging Cooper. When F2 regulations effectively became Formula One rules in 1961, Parnell continued with his Lotus 18, grouping with Gerry Ashmore and Andre Pilette as one of a trio of privateer entrants. His two World Championship starts that year produced tenth place at Monza and a retirement at Aintree. A crash at Brands Hatch at the start of 1962 curtailed his own driving, and his last race came in a Lotus-BRM Type 24 at the non-championship Austrian Grand Prix on the old Zeltweg airfield, where he finished an unclassified sixth.
In January 1964 Reg Parnell died unexpectedly at 53 following complications from an appendectomy. Tim took over the management of Reg Parnell Racing, which had recently acquired Lotus 25 Formula One cars — the same cars that had delivered Jim Clark his 1963 World Championship — combined with BRM V8 engines. The team fielded cars for Chris Amon, Mike Hailwood, Richard Attwood, Innes Ireland, and others. Amon's best result for the team was fifth in the 1964 Dutch Grand Prix. Hailwood, transitioning from motorcycle racing, scored championship points at Monaco despite crashing in practice the day before the race.
BRM approached Parnell to manage the team's Tasman Series campaigns over the winter months of 1965-66 and 1966-67. Drivers included Graham Hill, Jackie Stewart, Richard Attwood, and Piers Courage. These Tasman campaigns led naturally to Parnell Racing becoming BRM's junior Formula One team, running P126 cars for Piers Courage and Chris Irwin.
When Tony Rudd left for Team Lotus, Tim Parnell became the main BRM team manager. Under his direction the team fielded drivers including Pedro Rodriguez, Jo Siffert, Peter Gethin, Jackie Oliver, Howden Ganley, Jean-Pierre Beltoise, and a young Niki Lauda. Tony Southgate designed the cars that won several Grands Prix during this period, notably Siffert's win in the Austrian Grand Prix and Gethin's victory at Monza — for many years the closest and fastest Grand Prix finish in history. Beltoise won the 1972 Monaco Grand Prix in the wet for BRM, an unexpected result that received worldwide television coverage and delighted the Marlboro sponsors. BRM had become the first F1 team to carry full Marlboro sponsorship.
When Sir Alfred Owen suffered a stroke and his son David chose not to continue funding the team, Tim Parnell left BRM at the end of 1974, having grown weary of life under the regime of Louis Stanley.
After leaving BRM, Parnell managed Oulton Park and Mallory Park for Motor Circuit Developments, then served as General Manager of Donington Park. In 1995 he became an active director of the British Racing Drivers' Club, a role he held until 2003 when he became a Vice-President until 2010. He was elected a Full Member of the BRDC in 1962 and subsequently became a Life Member. He died on 5 April 2017 at the age of 84.