The son of two wealthy American artists, Tompkins knew Italy from a young age. When the war broke out he worked as a correspondent and in 1942 enlisted in the OSS, requesting action in Italy. The OSS had both espionage and sabotage tasks and functions to support partisan forces by sending money and weapons and identifying enemy targets for destruction. Arriving in Italy after Sept. 8, Tompkins made an agreement with Raimondo Craveri, the son-in-law of Benedetto Croce, to set up the Italian Resistance Organization (ORI), an intelligence body under American command.

Leaving Capri, Tompkins reached Corsica, thence the Lazio coast and arrived in Rome on January 21, 1944, just hours before Operation Shingle (code name for the Anzio landing) was triggered.

Despite objective difficulties, Tompkins managed to forge relationships with Roman resistance leaders, meeting several times with communist Giorgio Amendola, socialist Giuliano Vassalli and shareholder Riccardo Bauer. He carried out his activities mainly with the cooperation of the socialist partisan military organization, making use of the future ambassador Francesco Malfatti di Montetretto, thanks to whom it was possible to set up a secret information-gathering network, consisting of about sixty men who, around the clock, monitored the movements of German troops in and out of Rome on the consular routes. This enabled Tompkins, with the cooperation of Lieutenant Giglio and "Radio Vittoria" operators, to keep the Anglo-American contingent at the Anzio beachhead informed at all times with reliable news.

Unfortunately, Giglio, as a result of a denunciation, was captured by agents of the Koch Gang and then slaughtered at the Fosse Ardeatine, along with 14 other Tompkins collaborators.

For most of his mission, Tompkins hid in a secret room inside the ancient Palazzo Lovatelli in Rome's eponymous square. He assumed fictitious names and identities, in part to deceive the fascist services.

On June 4, 1944, as American columns were about to make their entry into Rome, Tompkins drafted and signed "the official order to assume control of public order." After the war, he devoted himself mainly to his work as a columnist, working with the CBS and the "New Yorker."