An engineering apprentice, Southgate learned about racing car design as a member of the 750 Motor Club, building his own car which was powered by an old Austin 7 engine. He was hired to work as a draughtsman with Eric Broadley's Lola Cars in January 1962 and worked on sportscars, Formula 1 and Indycar designs. He moved to Brabham for a year but went back to Lola to work with Broadley on the T70 sportscar which was successful in CanAm with John Surtees winning the inaugural title. This led to an offer from Dan Gurney to work for his Eagle operation in Santa Ana, California and Southgate designed a successful Eagle for Formula 5000 and Bobby Unser's Indianapolis 500-winning car in 1968.Southgate then returned to Britain to work as the chief designer at BRM, designing for the 1970 season. The cars were driven by Jackie Oliver and Pedro Rodriguez and were not very reliable but at Spa Rodriguez gave BRM its first victory for four years. He added a second place finish at the US GP at the end of the year. For 1971 he built the P160 for Rodriguez and Jo Siffert. Siffert won the Austrian GP in August and Peter Gethin - who replaced Rodriguez after he was killed in a sportscar race in July - won at Monza. The team finished second in the Constructors' Championship (with half the number of points scored by Tyrrell) but the year ended sadly with Siffert being killed in a non-championship F1 race at Brands Hatch in November.For 1972 Southgate revised the P160 and Jean-Pierre Beltoise used one to win his only F1 victory, in the rain at Monaco, while a variety of others struggled with the car and with its successor the P180.At the end of the year Southgate went to work for Don Nichols at Shadow, who had announced plans to build F1 cars in 1973. Southgate built the prototype Shadow DN1 in his garage at home in Lincolnshire and the car was quite competitive in the hands of George Follmer and Jackie Oliver. At the same time the CanAm Shadow DN2 finished 1-2 in the series that year.For 1974 Southgate built the DN3 for Peter Revson and Jean-Pierre Jarier. After a promising start to the year disaster struck at Kyalami with Revson being killed in testing when his suspension failed. The DN5 followed in 1975 with Jarier and a rising star called Tom Pryce who scored several promising results. At the end of the year Southgate built the Matra-engined DN7 before leaving the team when it emerged that sponsor UOP was not going to continue in 1976. Southgate moved to Team Lotus where he worked on the design of the Lotus 77 and with Peter Wright on the development of the Lotus 78. In mid-1977, however he returned to Shadow.At the end of the year he was one of the group which left the team to establish Arrows, the five founding members being Franco Ambrosio (A), Alan Rees (R), Jackie Oliver (O), Dave Wass (W) and Southgate (S). The team set up in headquarters at Milton Keynes and built the first Arrows F1 car. This was a copy of the Shadow DN9 and Shadow successfully sued the new team for copyright infringement. This meant a second car had to be built.In the year that followed Southgate built the Arrows A2 and A3 designs before leaving the team and doing freelance design, notably with Chevron.In 1980 he joined Theodore and designed the TY01 for the 1981 F1 season. At the end of 1982 the team merged with Ensign and Southgate decided to go into business with fabricator John Thompson to form Auto Racing Technology. They revamped the Osella FA1D in early 1983 but then became involved with two major Ford programs: the C100 sportscar and the RS200 rally car. Both were canceled before they were able to produce any major results.In October 1984 Southgate began working with Tom Walkinshaw Racing, designing the Jaguar sportscars which enjoyed enormous success in the late 1980s with victories at Le Mans in 1988 and 1990, three wins in the Daytona 24 Hours and several sportscar world titles.Southgate left Jaguar in 1991 but has continued to design sportscars for Toyota, Ferrari, Lister, Nissan and Audi.