Complex sentence: She was a major Hollywood sex symbol of the 1950s and early 1960s and 20th Century Fox's alternative to Marilyn Monroe who came to be known as the "Working Man's Monroe".
Split into simpler sentences: She was a major Hollywood sex symbol of the 1950s and early 1960s. Mansfield was 20th Century Fox's alternative to Marilyn Monroe and came to be known as the "Working Man's Monroe".