Koster was introduced to movies about 1910 when his uncle opened a very early movie theater in Berlin. Koster's mother went there every day to play the piano to accompany the films, and the young boy went with her. (Day care was non-existent then.) And Koster had to sit for a couple hours a day staring at the movie screen. Koster achieved fame as short story writer at age 16 or 17 and was subsequently hired by Berlin movie company as scenarist, became assistant to director Kurt Bernhardt. Bernhardt became sick one day and asked Koster had to take over as director. In about 1931 or 1932, Koster directed two or three films Berlin for UFA, when Hitler became president.
Koster, who was in the midst of directing a film, had already been subject of anti-Semitism, and knew he had to leave. He lost his temper at SS officer at his bank during lunch hour, and knocked the officer out. He went directly to railroad station and left Germany for France, where he was rehired by Bernhardt (who had left earlier). Eventually Koster went to Budapest and met and married Kato Kiraly in 1934. In Budapest he met Joe Pasternak who represented Universal in Europe, directed three films for him. In 1936 Koster got a contract to work with Universal in Hollywood, and he travelled to the United States to work with Pasternak, other refugees and his wife. After troubles with the studio (Koster didn't speak English) he convinced studio to let him make "Three Smart Girls" with Deanna Durbin. He coached Durbin, who was then 14 years old, himself.
This picture, a big success, pulled Universal out of bankruptcy. Koster's second Universal film, "100 Men and a Girl", with Durbin and Leopold Stokowski put studio, Durbin, Pasternak and Koster on top.
Koster went on to do numerous musicals and family comedies during late 1930's and early 1940's, many with Betty Grable, Durbin and other musical stars of the era. Koster worked at Universal until 1941 or so, and then moved to MGM, and then Fox in 1948. Ironically, despite his escape from Nazi Germany, when the United States entered World War II Koster was considered an enemy alien and had to stay in his house in the evening. Actor Charles Laughton would visit Koster and play chess with him.
When he married his second wife, Peggy Moran, in 1942 he promised her he would put her in every movie he made from then on. He did, but it was her statue. Usually it is a sculptured head on a mantlepiece or a piano or desk. In "The Robe" he commissioned a Grecian bust which appears prominently in a Roman villa.
Koster discovered Abbott and Costello working at a nightclub in New York. He returned to Hollywood and convinced Universal to hire them. Their first picture, which featured the "Who's on First" routine, was "One Night in the Tropics". The female lead in that picture was Peggy Moran who would later marry Koster. At the time they did not know each other.
Koster was nominated for an Academy Award for "The Bishop's Wife". He directed Richard Burton's first U.S. film, "My Cousin Rachel", and then was given first CinemaScope film to direct, "The Robe" in 1952. He did a couple more costume dramas, including "Desiree" with Marlon Brando, then went back to family comedies and musicals, including "Flower Drum Song" for Universal in 1961. His last picture was the "The Singing Nun" in 1965. Retired to Leisure Village, Camarillo, California, to indulge in his lifelong interest in painting. Did a series of portraits of the movies stars with whom he worked.