Between 1950 and 1971, Koch appeared in more than 65 films. In the haunting 1954 espionage thriller Night People she starred alongside Gregory Peck. Sergio Leone's 1964 production A Fistful of Dollars showcased her alongside Clint Eastwood as a civilian tormented by ruthless local Gangsters, torn between her husband and child and the villains. In Germany she was probably best-loved for her many years of participation in the highly popular TV game show Was bin ich? which ran from the 1950s until 1988 and achieved ratings of up to 75% at its peak.

In 1953, she married the physician Gerhard Freund, with whom she has two sons. The marriage ended in 1973 after Freund began an affair with Miss World 1956, Petra Schürmann, whom he later wed.

In 1971, she resumed the medical studies she had broken off in the early 1950s to become an Actress. She earned her MD in 1974 and practiced Medicine until 1997 as a specialist in Munich. Also in 1974, she was one of the initial hosts of Germany's pioneering talk show 3 nach 9 (de) (Three After Nine), for which she was awarded one of the most prestigious awards of the German television industry, the Grimme-Preis. She also hosted other television shows and, as of 2014, still has a medical advice program on radio.