The daughter of Irish immigrants, Moriarty was raised in Westchester County, New York and began acting in local dinner theaters while still in high school. She was only 17 years old when she first auditioned for the role of Vicki La Motta and after several attempts and a screen test which convinced director Scorsese and co-star Robert De Niro was hired for the part. Projecting a world-weariness that belied her youth, many thought her to be much older. Casting agents responded in kind by pigeonholing her as either maternal figures or generic "tough broads" by the 1990s (i.e., the missing witness in "Another Stakeout" 1993, the blowzy mom in "A Brother's Kiss" 1997). Even when she tried her hand at TV series, she was cast as a caustic but loving matriarch in the short-lived "Bless This House" (CBS, 1995).

Moriarty did excel at playing over-the-top comic villains as she demonstrated with her nasty heiress in "Casper" (1995) and her homophobic camp counselor attempting to rehabilitate gay and lesbian youth in "But I'm a Cheerleader" (2000). Occasionally, a director would break the mold, as James Mangold did in casting her as Harvey Keitel's wife in "Cop Land" (1997) but most of her roles were of the ilk of her madam in "Gloria" or her mobster's wife in "New Waterford Girl" (both 1999). She herself is cognizant of the fact that with her smoky voice and sexy allure she would hardly be cast as a wallflower, yet one cannot help but wonder what might happen should an imaginative casting agent or filmmaker offer her a part commensurate with her untapped talents. Moriarty continued to specialize in over-the-top villainesses but also appeared in the occasional serious supporting turn. She was reunited with her "Raging Bull" co-star De Niro in the comedy "Analyze That" (2002).