Macklin was born in Kensington, and educated at Eton College. His father, Noel Macklin, founded the Invicta and Railton car companies, and Fairmile Marine, a manufacturer of motor gun and torpedo boats during World War II. He volunteered for service with the Royal Navy in 1939, and was assigned to work on motor gun boats in line with his father’s business interests.
Following demobilisation after the Second World War, Macklin pursued a career as a racing driver. He secured an entry to the 1948 Grand Prix des Frontières, practising by driving his Invicta at high speeds on public roads, even teaching himself to four-wheel drift around Belgrave Square in London’s Mayfair. He impressed in the race and earned a signing with Aston Martin, becoming a reserve driver for Le Mans and racing at the Spa 24 Hours, finishing fifth.
Macklin finished fifth at Le Mans in 1950 alongside teammate George Abecassis. Abecassis invited him to join HWM for several races, culminating in his victory in the 1952 BRDC International Trophy. HWM also gave Macklin his debut in the Formula One World Championship, though the team was not competitive against better-funded entries and he scored no world championship points. While at HWM, Macklin formed a close bond with Stirling Moss. He returned to Le Mans with Aston Martin in 1951, finishing third overall and taking his second S3.0 class victory. He left Aston Martin in 1952, dissatisfied with his retaining fee, and joined Bristol for the following year, though the team found little success.
Macklin was infamously involved in the 1955 Le Mans disaster, the most catastrophic accident in racing history, which killed Pierre Levegh and 83 spectators. He swerved to avoid hitting the Jaguar of Mike Hawthorn, who was braking hard in a late attempt to pit, and moved into the path of Levegh’s car, causing it to clip his. Macklin was uninjured in the crash but was deeply affected by the incident, feeling that Hawthorn attempted to alleviate responsibility and that the racing community blamed him. Another tragic experience followed at the Tourist Trophy at Dundrod, where he crashed his Austin-Healey 100S avoiding an accident in which Jim Mayers and William T. Smith were killed.
Following the Dundrod crash, Macklin retired from motor sport at the urging of his then-girlfriend. He later moved to Spain before returning to England when he became ill. He died on 29 August 2002 in Tenterden, Kent, four days before his 83rd birthday.
After retiring from racing, Macklin joined Facel Vega in Paris, running the export division until the company failed in 1963. He then began working for London car dealership H.R. Owen.
Macklin was married twice, firstly to Shelagh and subsequently to Gillian. He had two children from his first marriage and one from the second. George Abecassis had been critical of Macklin's attitude, stating: "He never cared whether he started in a race or not... Sometimes it was a nightmare to make him practice at all. If there was some blonde he was after he just wouldn't show up."