He briefly served as General Officer Commanding (GOC) of the 44th (Home Counties) Division, a Territorial Army (TA) formation, from April to June 1941, before being appointed Head of the British Military Mission in Moscow, shortly after the German invasion of the Soviet Union. Appointed on 24 June 1941 and arriving in Moscow on 14 July 1941 to be greeted as a guest of honour by Stalin at the Kremlin, Mason-MacFarlane had great hopes of Anglo-Soviet cooperation, which were soon dashed as he learned that the xenophobic Soviet regime regarded him as a spy. Unlike the other British generals who expected the Soviet Union to collapse in the summer or autumn of 1941, Mason-MacFarlane had a higher opinion of the Red Army and consistently predicted that the Soviets would hold out. "Mason-Mac" described the morale of ordinary Russians as high, reporting to London that they were just as committed to victory as ordinary British people. Though relations with his Soviet counterparts were difficult, Mason-MacFarlane found ordinary Russians friendlier and he worked hard to assist the Soviets. He played a crucial role in negotiating the transfer of the so-called Anders' Army, made up of the surviving Polish POWs taken prisoner by the Red Army in 1939, who in March 1942 crossed over from the Soviet Union to Iran and from there travelled on to join the British 8th Army.