Judd began his working life shortly after leaving school, securing a position at Coventry Climax, then one of Britain's foremost engine manufacturers. The company supported his further education through a local college scholarship. In the early 1960s he worked on the Coventry Climax Formula One engine being used at the time by Jack Brabham, gaining hands-on experience with the competitive front-line power units of the era.
When Jack Brabham left Coventry Climax for Repco, Judd followed, hired to work alongside engine designer Phil Irving on the new 3-litre Brabham-Repco engines. The results were emphatic: the Repco-powered Brabham team won back-to-back Drivers' and Constructors' World Championships in 1966 and 1967. Brabham subsequently switched to Ford DFV engines built by Cosworth, and again enlisted Judd for developmental work on those units.
After the Brabham team was sold to Bernie Ecclestone, Jack Brabham turned his attention to a new venture in engine development. In 1971 he and Judd co-founded Engine Developments Ltd., which in the years that followed prepared and developed Cosworth DFV engines for teams including Williams, Arrows, Lotus, Ensign, and Fittipaldi.
In the late 1970s Judd and Engine Developments expanded their scope to include IndyCar racing. In 1980, Judd was approached by Honda to develop an engine for the manufacturer's return to Formula Two, a project undertaken in association with Ron Tauranac's Ralt team.
Engine Developments returned to the front of the F1 grid in 1988, supplying engines for Williams, Ligier, and March. In 1991 Judd struck a supply agreement with Scuderia Italia, providing V10 engines for that team's Dallara-designed chassis.
In 1992 Judd entered into a collaboration with Yamaha to develop and build engines specifically for Formula One. The partnership enjoyed its finest moment at the 1997 Hungarian Grand Prix, where Damon Hill drove a Yamaha-engined car to second place behind race winner Jacques Villeneuve. The Yamaha partnership concluded at the start of the 1998 season, after which Judd continued independent development of his V10 engine through 1998 and 1999.
From the early 2000s, Judd and Engine Developments shifted focus toward touring cars and sportscar racing. The move paid dividends quickly: the company secured victory in the 2001 American Le Mans Series with Dick Barbour Racing, and followed that with second place in the teams' championship of the Rolex Sports Car Series in 2002 with Doran.
In 2004 a collaboration with Ray Mallock Ltd. brought further recognition, with the XV675 engine winning the LMP2 class at the 2005 24 Hours of Le Mans, one of the most prestigious results in the company's sportscar history. By 2007 Judd planned to continue supplying an LMP2 team with his engines, demonstrating the durability of Engine Developments' relevance across multiple racing disciplines.
John Judd's career traces an arc through the most significant eras of post-war motor racing. From championship-winning Repco-Brabham engines in the 1960s to Le Mans class victories four decades later, Engine Developments remained competitive across a remarkable spread of categories and regulations. Judd's ability to adapt — moving from F1 to IndyCar to sportscar racing, and from customer preparation work to original engine design — marks him as one of Britain's most quietly consequential racing engineers.