Welch founded Denis Welch Motorsport in 1976–77 at a petrol station and garage in Yoxall, Staffordshire, initially concentrating on performance parts for Austin-Healey models. His family background in engineering ran deep: his great-grandfather, John Walter Brooke, founded J.W. Brooke & Co. in Lowestoft in 1874, later producing one of the UK's first six-cylinder car engines in 1904 and developing marine racing engines.
The business expanded over four decades into race preparation, restoration and comprehensive parts manufacturing, supplying customers across the UK, Europe, Australia and the Far East. Key specialisations included Austin-Healey 3000 and 100 series, Jaguar E-Type, Lotus, MG and Formula Junior vehicles. The company's performance catalogue for Austin-Healey models — covering CNC-machined aluminium cylinder heads, high-performance camshafts, roller rockers and valvetrain packages — became an essential reference for historic competitors worldwide.
Welch's son Jeremy took over the business in 2006–07 and has continued to develop it, investing in Haas CNC machinery and expanding in-house manufacturing capability. As recently as 2024, the company prepared the engine, gearbox and axle for the Tour Auto-winning Jaguar E-Type.
Welch started racing in 1965. He took competition seriously but focused his professional energies on building the engineering business, racing on the side throughout his working life. In 1979 he purchased a 1959 Austin-Healey 3000, fully restored it, and began campaigning it in historic sports car events. Nicknamed The Bulldog and carrying the registration 6200 NO, the car was painted red and became his signature mount, driven in a flamboyant, oversteering style.
With The Bulldog, Welch won the HSCC Pre-1960 Historic Sports Car Championship in both 1986 and 1987, and the HSCC Classic Sports Car Championship in 1988, 1991 and 1992. Victories came at the Eifel Classic at the Nürburgring Nordschleife and at events at Bathurst. He and son Jeremy drove the car to a narrow second at the 1997 Motor Classic Six Hours of Spa, losing the lead only in the final half-hour when a backmarker cost them a penalty. Further race wins at Brands Hatch in 1993 and Donington Park in 1996 received magazine coverage.
Welch took up historic Formula Junior racing, driving a Merlyn in the FIA Lurani Trophy — the first officially sanctioned historic motorsport series. His first single-seater race weekend in 25 years came at Donington Park in 1995, where he finished second and third. He never won the Lurani Trophy title but lost out on a tie-break in 2004.
His Formula Junior victories included events at Dijon-Prenois (2001), the Nürburgring (2001 and 2004), Pau (2001), Donington Park (2004 and 2005), Silverstone (2008), and Monza in 2004 — where the margin of victory was 0.003 seconds, the closest finish ever recorded at the circuit. Most prized were three victories in the Formula Junior race at the Historic Grand Prix of Monaco: 2000, 2002 and 2006. The 2000 win was his first in the category and he was visibly moved afterward: "I've waited a long time for a Junior win, so this is magic. I'd never classed myself as that good a driver, but to see Stirling Moss half out of his car, waving and cheering as I came in, was incredible."
In 2003 Welch finished third in both the GP Anniversary Trophy and the Golden Jubilee Cup at the Macau Grand Prix, driving a Lotus 23B — the same car he had won with at Donington Park earlier that year. He also won at Estoril in 2004 in an Elva GT car.
On 27 July 2014, Welch was fatally injured during the opening lap of the HGPCA Jack Brabham Memorial Trophy race for pre-1966 Grand Prix cars at the Silverstone Classic. He had entered a 1960 Lotus 18 that had previously been raced in South African Formula One by Syd van der Vyver. Gearbox problems during qualifying had sent him to the back of the grid. Two cars immediately ahead of him collided at the corner between the Village complex and The Loop; Welch could not avoid them, his Lotus clipped a competitor's car and rolled over. He was transported to the circuit's medical centre and died from his injuries.
The accident was the first fatality in the Silverstone Classic's 24-year history. His wife Tina, with whom he had been married for 45 years, was watching from the pits at the time. His son Jeremy reflected: "The one consoling fact was that it was very swift and he died doing what he loved." Former rival Mark Gillies described Welch as "a fantastic driver, especially in the big Healeys — a tough competitor, a real enthusiast and all-around good guy."
Denis Welch Motorsport continues to operate from Yoxall under Jeremy's leadership, preserving both the business and his father's legacy in historic motorsport.