Though most famous as the fourth and final wife of actor Peter Sellers, she was also an actress in her own right. Her screen credits include roles in such 1970s films as Schizo, A Long Return, and Phase IV.
At the age of sixteen, she played the role of Mary Custance in the science fiction feature No Blade of Grass.
Her final acting role was in the 1979 movie The Prisoner of Zenda.
Her marriage to Peter Sellers lasted from 1977 until his death in 1980, after which she was briefly married to comedian and television presenter David Frost. Before ending in divorce in the early 1990s, her third marriage, to American heart surgeon Barry Unger, produced a daughter named Cassie.
In the 1972 movie version of Henry VIII and His Six Wives, Frederick portrayed the king's fifth wife, Catherine Howard, and actress Charlotte Rampling played Henry VIII's famous second wife, Anne Boleyn.