[4][5] During World War II, Grover-Williams, code named Sebastian, worked as an agent in France for the clandestine British Special Operations Executive (SOE) organization.

[7] Mechanically inclined and fascinated by motorised vehicles, at the age of 15 Grover-Williams acquired an Indian motorcycle and it became his pride and joy.

[4] That same year, driving a Bugatti 35B, painted in what would become known as "British racing green", he won the inaugural Monaco Grand Prix beating the heavily favoured Mercedes of the German driver, Rudolf Caracciola.

[4][5] Successful financially, Grover-Williams and his wife maintained a home in a fashionable district of Paris while owning a large house in the resort town of La Baule, Pays de la Loire, on the Bay of Biscay, which was home to one of the annual Grand Prix races.

[14] On 29 May 1942, Grover-Williams and another SOE agent, Christopher Burney, parachuted "blind" (with no reception committee on the ground) into France near Le Mans.

[15] Grover-Williams proceeded to Paris where he reunited with his wife, Yvonne, but, for security reasons, he found himself a different place to live and work.

Grover-Williams recruited two fellow race-car drivers, Robert Benoist and Jean-Pierre Wimille, into his network and the trio used their wives and other women as couriers.

This problem was partially solved in January 1943 when Grover-Williams established contact with Jack Agazarian, the newly arrived radio operator for the Prosper Network.

Starting in mid-February, Grover-Williams organized with SOE six parachute drops of weapons and other supplies for the French resistance.

[18][19] Grover-Williams' downfall began 31 July 1943 when a German direction-finding van pinpointed the location of Dowlen and his radio and arrested him.

[20][21] When Grover-William's wife, Yvonne, learned of his arrest, she fled Paris and took refuge with friends in the village of Thorenc.

In March 1945, the Nazi government decreed "special treatment" for a list of political prisoners, including Grover-Williams.

[26] After the war, Grover-Williams' widow, Yvonne, eventually settled in the village of Amfreville-la-Campagne in Normandy where she managed a dog kennel breeding terriers.

[27] Robert Benoist's granddaughter, among others, claimed that William Grover-Williams survived the war and lived with Yvonne from 1948 until her death under the assumed identity of "Georges Tambal."

[27] The Saboteur, a 2009 video game, features an Irish protagonist named Sean Devlin who is inspired by Grover-Williams.