Frost began his aeronautical career in the 1930s as an apprentice for Airspeed Limited before he moved on to the Miles, Westland, Blackburn and Slingsby companies. In 1937, Frost had designed the fuselage of the new Westland Whirlwind fighter. At Blackburn, he had been involved with the design and construction of their pre-war wind tunnel. While working for Slingsby Sailplanes from 1939 to 1942, he met his future wife, Joan, who had worked in the Slingsby Design Office as a technical artist. Frost designed the Slingsby Hengist, a troop-carrying glider to be used for the Normandy landings. It was not a success and only a few were built but it included an ingenious innovation: the use of a rubber bag undercarriage.