Born in London, Simmonds began his motorcycle racing career in 1960 riding a 50cc Itom motorcycle. By 1963 he had progressed sufficiently to win the 125cc British road racing national championship on a Tohatsu, which he continued to race alongside his brother Mike. His results attracted the attention of the Kawasaki factory, earning him an invitation to race one of their new 125cc machines at the 1966 Japanese Grand Prix.
Simmonds persuaded Kawasaki management to loan him a motorcycle for the 1967 125cc Grand Prix world championships. The task was immense: the Kawasaki KA-1 125cc twin-cylinder was clearly outclassed by the expensive multi-cylinder machinery fielded by the dominant Yamaha and Suzuki factory teams. Without financial or mechanical backing from Kawasaki — relying only on occasional spare parts sent from Japan — Simmonds spent the 1967 and 1968 seasons methodically solving the machine's reliability problems.
His persistence paid off when the FIM announced new regulations effective from 1969, restricting 125cc machines to two cylinders and six-speed transmissions. The rule change caused Yamaha and Suzuki to withdraw their factory teams entirely. Simmonds and his now-competitive Kawasaki were perfectly positioned to capitalise, and he won the 1969 125cc World Championship impressively, failing to finish first or second in only a single race across the entire season. The victory was the first world championship win in history for Kawasaki.
Competition increased in 1970 and Simmonds dropped to fourth in the 125cc standings, though he still won the Finnish Grand Prix and added podium finishes at the Dutch and Belgian events. In 1971 he finished sixth in the 125cc championship, adding a win at the German Grand Prix at Hockenheimring.
In 1971, Simmonds also tackled the 500cc premier class on a Kawasaki H1R. Dissatisfied with the machine's handling, he had it rebuilt around a frame designed by Ken Sprayson, greatly improving its behaviour. The effort delivered results: he won a pre-season invitational 500cc race at Mettet, finished second to Giacomo Agostini at the Finnish Grand Prix, and collected third places in Holland and Italy. At the season-ending Spanish Grand Prix at Jarama — with Agostini absent after already securing the title — Simmonds took a 500cc class victory, marking Kawasaki's first-ever premier-class Grand Prix win. He ended the year fourth in the 500cc World Championship despite missing four rounds.
In 1972, seven years after his first appearance on the 125cc Kawasaki, Simmonds rode the machine to a remarkable third place at the Dutch TT.
In October 1972, Simmonds attended a non-championship race at Rungis near Paris. While there, a gas cylinder in a caravan belonging to fellow racer Jack Findlay exploded and caught fire. Believing Findlay's mother was trapped inside, Simmonds rushed to help at the moment the cylinder exploded and was engulfed in flames. He died from his injuries just two days before his thirty-third birthday.
Dave Simmonds is remembered as a racer of extraordinary self-reliance, who took a works-unsupported Kawasaki from also-ran to world champion through his own mechanical skill and determination. His 1969 championship remains Kawasaki's first world title and one of the most celebrated underdog achievements in Grand Prix motorcycle racing history. His death at 32 robbed the sport of a rider who had already demonstrated genuine capability in both the 125cc and premier 500cc classes.