He was soon cast in his first British movie, playing a murderous actor in Train of Events (1949). Critic C. A. Lejeune praised his work in the London Observer commenting that he "adds good cheekbones to a quick intelligence and is likely to become a cult, I fear." The Scotsman said "he should be regarded as one of the most hopeful recruits to the British screen."
During his war service Finch was given leave to act in radio, theatre and film. He appeared in a number of propaganda shorts, including Another Threshold (1942), These Stars Are Mine (1943), While There is Still Time (1943) and South West Pacific (1943), the latter for Ken G. Hall. He also appeared in two of the few Australian feature films made during the war, The Rats of Tobruk (1944) and the less distinguished Red Sky at Morning (1944).
Finch returned to Australia to make The Shiralee (1957), one of his favourite parts, and the tenth most popular movie at the British box office that year. He followed it with another Australian story, the bushranger tale Robbery Under Arms (1957), which did less well. However exhibitors still voted Finch the third most popular British star of 1957, and the fifth most popular overall, regardless of nationality.
He had a small role as an Australian prisoner of war in The Wooden Horse (1950), the third most popular film at the British box office in 1950. His performance as a Pole in Daphne Laureola led to his casting as a Polish soldier in The Miniver Story, the sequel to the wartime morale boosting film Mrs. Miniver; unlike its predecessor, it was poorly received critically.
During this time, Finch continued to appear on stage in various productions while under contract to Olivier. Finch's closeness to the Olivier family led to an affair with Olivier's beautiful but increasingly unstable wife, Vivien Leigh, which began in 1948, and continued on and off for several years, ultimately falling apart due to her deteriorating mental condition.