Judd left school at a young age and secured employment at Coventry Climax, where he received a scholarship to study at a local college. In the early 1960s he worked on the Coventry Climax Formula One engine used by Jack Brabham. When Brabham moved from Coventry Climax to Repco, Judd followed and was tasked with working alongside engine designer Phil Irving to develop the new 3-litre Repco engines.
The engines Judd helped develop for Brabham powered the team to drivers and constructors championships in both 1966 and 1967, with Jack Brabham winning the former and Denny Hulme the latter driver title. When Brabham subsequently switched to Cosworth DFV power, Judd again remained with the team in a developmental capacity, working on the Cosworth units. After Brabham sold the team to Bernie Ecclestone, Judd and Brabham co-founded Engine Developments Ltd. in 1971. In the years following, the company prepared and developed Cosworth engines for teams including Williams, Arrows, Lotus, Ensign, and Fittipaldi.
In the late 1970s, Engine Developments expanded into IndyCar racing. In 1980, Honda hired Judd to develop a power unit for the company's return to Formula Two in conjunction with Ron Tauranac's Ralt team.
Engine Developments returned to the Formula One grid as a constructor-supplier in 1988, providing engines to Williams, Ligier, and March. In 1991 Judd struck a deal with Scuderia Italia to supply V10 engines for a Dallara chassis. In 1992 he entered a partnership with Yamaha to build and develop Formula One engines jointly. The collaboration's best result came at the 1997 Hungarian Grand Prix, when Damon Hill drove a Yamaha-engined car to second place behind Jacques Villeneuve. The Yamaha partnership ended at the start of the 1998 season, and Judd continued developing his V10 engine independently through 1998 and 1999.
In the early 2000s, Engine Developments shifted focus toward touring cars and endurance racing. The company achieved victory in the 2001 American Le Mans Series with Dick Barbour Racing, and finished second in the teams championship of the Rolex Sports Car Series in 2002 with Doran. A collaboration with Ray Mallock Ltd. led to the Judd XV675 engine winning the LMP2 class at the 2005 24 Hours of Le Mans, one of the most prominent results in Engine Developments' endurance racing programme.
John Judd's career represents a through-line from the Repco championship era of the mid-1960s to the endurance racing circuits of the 2000s. Engine Developments remained one of a handful of British independents capable of competing with factory programmes across multiple formulae. Judd's contribution to the Repco engines that gave Brabham two championships and his later work supplying a range of Formula One teams places him among the significant — if less publicly prominent — figures in postwar British motorsport engineering.