Before his marriage to Palmer, Thompson had a relationship with Mexican actress María Félix. They met in Argentina in 1952 during the filming of Luis César Amadori’s La pasión desnuda. The relationship was reportedly serious to the point that they became engaged to be married, and Félix even called for her son Enrique Álvarez Félix to meet his potential stepfather. However, during filming, Félix received a call from director Emilio Fernández to offer her the leading role in The Rapture, which Félix accepted, telling Thompson that after filming of La pasión desnuda ended she would return to Mexico to promote the film and announce their future marriage. However, once in Mexico, Felix canceled the wedding days before it would take place, stating that she concluded that the only thing that united her to Thompson was a mere physical attraction and not true love, before marrying The Rapture’s leading man Jorge Negrete.

Although references to him recorded in the diaries of Noël Coward suggest that he was bisexual, Thompson married German-born actress Lilli Palmer shortly after her divorce from Rex Harrison in 1957. They remained married until her death in 1986.

He moved to Europe and appeared in a large number of German films. He was chiefly known to English speakers for his appearance as Carlos Varela in the 1963 ITC Entertainment series The Sentimental Agent. In the late 1960s, Thompson left acting to become a writer and TV producer.

His first success on the European book market was The Assassination of Winston Churchill (1969), a refutation of allegations by David Irving (Accident. The Death of General Sikorski, 1967) and the German playwright Rolf Hochhuth (Soldiers, premiered in the UK in 1968, London) that war time premier Winston Churchill had a part in the death of Polish General Władysław Sikorski, who perished in an air plane crash at Gibraltar on July 4, 1943, allegedly due to sabotage.