Born in London, Simmonds began racing motorcycles in 1960 on a 50cc Itom. By 1963 he had won the 125cc British road racing national championship on a Tohatsu, competing alongside his brother Mike. His strong domestic results attracted the attention of the Kawasaki factory, which invited him to race one of their new 125cc machines at the 1966 Japanese Grand Prix.
Impressed by the opportunity, Simmonds persuaded Kawasaki management to loan him a motorcycle for the 1967 Grand Prix world championship season. The Kawasaki KA-1 125cc twin-cylinder machine was outclassed by the multi-cylinder works bikes fielded by Yamaha and Suzuki — the RA31 V4 and RT67 twin respectively — and Simmonds ended the year seventh in the championship. Over the 1967 and 1968 seasons, operating largely without financial or mechanical backing from Japan, he methodically worked through the Kawasaki's reliability problems using only occasional shipments of spare parts from the factory.
The turning point for Simmonds came from an unexpected source: FIM regulation changes that took effect in 1969. In an attempt to curb escalating costs, the governing body limited 125cc machines to two cylinders and six-speed gearboxes. Yamaha and Suzuki, unwilling to homologate their sophisticated multi-cylinder designs to the new rules, withdrew their factory teams from the class. Suddenly, Simmonds and his well-sorted, regulation-compliant Kawasaki were competitive.
He seized the opportunity decisively. In the 1969 125cc World Championship he was a consistent frontrunner, finishing first or second in all but one race. The title was his, and with it came a place in history as the rider who delivered Kawasaki their first-ever Grand Prix world championship.
Simmonds defended his title in 1970 but dropped to fourth place as competition increased from Dieter Braun on a Suzuki, Angel Nieto on a Derbi, and Borje Jansson on a Maico. He still managed a race victory at the Finnish Grand Prix and two second-place finishes in the Netherlands and Belgium.
In 1971 Simmonds expanded his programme to include the premier 500cc class, riding a Kawasaki H1R. Dissatisfied with the machine's handling, he had the H1R rebuilt around a Ken Sprayson-designed frame, significantly improving its behaviour on circuit. The results were notable: he won the pre-season Mettet Grand Prix, finished second to Giacomo Agostini at the Finnish Grand Prix, and took third places in the Netherlands and Italy. At the season-closing Spanish Grand Prix at Jarama — with Agostini absent, having already secured the championship — Simmonds won the race outright, giving Kawasaki their first premier-class Grand Prix victory. He ended the season fourth in the 500cc standings despite missing four rounds.
In 125cc racing in 1971, Simmonds secured a victory at the German Grand Prix at Hockenheim on his way to sixth in the final standings. In 1972, seven years after his first appearance on the Kawasaki 125, he rode it to a remarkable third place at the Dutch TT.
On 23 October 1972, Simmonds was attending a non-championship motorcycle race at Rungis, near Paris, when a gas cylinder exploded in a caravan belonging to fellow racer Jack Findlay. Believing that Findlay's mother was trapped inside, Simmonds ran to help at the moment of the explosion. He was engulfed in flames and died of his injuries two days before his thirty-third birthday.
Dave Simmonds is remembered as one of the most capable independent racers of the classic Grand Prix era. His 1969 title remains a landmark in Kawasaki's motorsport history — the result of years of patient, self-funded development rather than factory muscle. His later exploits in the 500cc class demonstrated that his ability extended well beyond the lightweight categories.