Marilyn lives with the Munsters but it is never made clear why Marilyn lives with the Munsters, rather than with her parents, who are said to be still in Transylvania. It is noted that the Munsters had raised Marilyn since she was fairly young. Although she is Lily's niece, (according to Herman, the daughter of Lily's sister), she is always referred to as "Marilyn Munster". The fractured familial ties were never explained (perhaps she escaped Hungarian tyranny, while her folks remained). Although Lily and her father the Count (AKA Grandpa) are vampire/ghouls, Lily's brother Lester is a werewolf. One can only guess at the nature of Grandpa's late wife/ Lily's mother. A photograph of a woman in European peasant garb was seen faintly in Marilyn's bedroom, suggesting it may be Marilyn's mother.

Marilyn is a fetching young blonde resembling Marilyn Monroe, and the only family member who is not ghoulish in appearance; by the Munsters' aesthetic standards, she is distressingly unattractive. The family views Marilyn's appearance as an affliction, but still treat her with kindness and love. Marilyn herself is all too aware of her "homeliness" and bemoans that she keeps scaring off potential boyfriends, having no clue that the young men are in fact frightened away by her monstrous family.

She attends Westbury College in Mockingbird Heights. She is devoted to her family, and the surest way for a suitor to alienate Marilyn (rather than vice versa) is to disparage them. She considers her Uncle Herman and Grandpa the "two [finest] men [who ever] walked this earth."

Despite being a platinum-blonde beauty, the Munsters' queer subculture of vampires, werewolves, zombies, sorcerers, mad scientists, et al., have made her a ghoul at-heart (she regularly assists Lily in cooking rodents, & festooning the house with cobwebs). Like the rest of her family, she believes that the Munster lifestyle is normal, and dissenters are out-of-step.