Costin worked as an engineer at the de Havilland Aircraft Company, where he acquired deep expertise in aerodynamics and lightweight construction. In his youth he had been an Olympic-standard swimmer, and in later life he would compose music — interests that pointed to a broad, unconventional mind. His brother Mike was a former de Havilland engineer who left to work for Lotus Engineering Ltd., and that sibling connection would eventually draw Frank into the world of motor racing.
In 1954, Mike Costin asked Frank to design an aerodynamic body for a new Lotus racing car. Intrigued by the prospect of bringing aircraft aerodynamic knowledge to bear on automobiles, Costin designed the body for the Lotus Mark VIII. He was never a formal Lotus employee; his contributions were made either as a paid consultant or as a volunteer.
The commission that cemented his reputation came in 1956 when Colin Chapman was asked by industrialist Tony Vandervell to design a Grand Prix car capable of challenging the Ferrari and Maserati dominance of Formula One. Chapman recommended Costin as the body designer for the new Vanwall. The resulting car, clothed in a remarkably clean and efficient aerodynamic envelope based on aerofoil cross-sections, became the most powerful British Grand Prix car of its era. In 1958 the Vanwall team took the inaugural Constructors' Championship, and Costin's bodywork was a key factor in that success.
Beyond aerodynamics, Costin explored the structural possibilities of plywood as a chassis material. Applying principles borrowed from aircraft construction, he built lightweight monocoque structures from plywood that were both stiff and exceptionally light — a decisive advantage in the lower-capacity sports car categories of the period. This thinking was radical at a time when spaceframe tubes dominated racing car construction.
Costin was also a co-founder of the Marcos car company, partnering with Jem Marsh of Speedex Cars — the name Marcos being formed from MARsh and COStin. The early Marcos cars were notable for their plywood monocoque chassis, a direct expression of Costin's aeronautical construction philosophy. He contributed to road car projects for Lister and Lotus, designed racecar chassis for Maserati and other constructors, and collaborated on an ultra-light glider with Keith Duckworth, a long-standing friend and his brother's business partner at Cosworth.
Later in his career Costin designed several road car projects under his own name, including the Costin Amigo, the TMC Costin, and the Costin Sports Roadster — vehicles that applied his hallmark combination of aerodynamic efficiency and structural ingenuity to road-going machines.
Frank Costin occupies a distinctive place in the engineering history of motor sport as one of the first practitioners to transfer systematic aerodynamic knowledge from aviation to racing cars at a time when most car designers regarded streamlining as little more than styling. His influence on the Vanwall — and through that car on the very first season of the Constructors' Championship — gives him an enduring claim on Formula One history. His plywood monocoque work predated and in some respects anticipated the composite chassis revolution that would come decades later. The Marcos marque, bearing his name, survived into the twenty-first century as a reminder of the structural and aesthetic ideas he pioneered in the 1950s.