[5] Hereward and his brothers Geoffrey and Ivon had a mechanical workshop at their home at the rectory in Crux Easton near Newbury.

De Havilland flew in various air campaigns in Europe and the Middle East in World War I and reached the rank of Major.

[12][13][14] He flew solo in the 1929 Western Australian Centenary Air Race in a modified de Havilland DH.60 Moth, named "Black Hawk", coming second on handicap and winning the £300 fastest overall time prize in 22 hours 50 minutes 23 seconds and averaging 107.8 miles per hour (173.5 km/h).

[15][16][17][18] The same year he joined the board of Airspeed Ltd. at Christchurch Airfield in Hampshire and was appointed Joint Managing Director.

[20][21] He retired as Managing Director of de Havilland's Airspeed Division in 1959, joined the board and became Deputy Chairman.