David Stirling, the founder of the SAS, recognized Lewes's potential and recruited him into L Detachment. Stirling sought individuals with intelligence, physical prowess, and a willingness to take risks, qualities Lewes possessed in abundance.

Lewes participated in the brutal and innovative training program devised by Stirling. This included physical endurance tests, weapons training, demolition techniques, and desert survival skills. The training was designed to prepare the men for operating independently behind enemy lines.

Recognizing the ineffectiveness of existing explosives for damaging armored vehicles, Lewes developed the 'Lewes Bomb,' a more effective incendiary device. This invention played a crucial role in the early SAS raids against Axis airfields in North Africa.