Original: Although he is probably best known today for his performance in Singin' in the Rain, he was a dominant force in Hollywood musical films from the mid 1940s until the demise of this form in the late 1950s.
Simplified: Although he is probably most famous today for his act in Singin' in the Rain, he was the most important actor in the Hollywood musical film industry from the mid 1940s until its decadence in the late 1950s.