Magnuson's first big-screen appearance was a small role in the crime thriller "Vortex" (1982); she went on to play similar parts in "The Hunger" (1983), "Perfect Strangers" (1983) and "Desperately Seeking Susan" (1985). She had a larger role in the fantasy "Sleepwalk" (1986), but was still largely unknown to the public-at-large until she starred opposite John Malkovich in Susan Seidelman's comedy "Making Mr. Right" (1987), about a scientist who builds an android boyfriend. From 1989 through 1992, she played an incorrigibly chic magazine editor in TV's critically-acclaimed sitcom "Anything But Love" (ABC), starring Jamie Lee Curtis and Richard Lewis. Since then, Magnuson has appeared on many TV specials (including, bizarrely, as a judge for the 1995 Miss Universe Pageant) and the ABC-TV movie "The Barefoot Executive" (1995).
Onscreen, Magnuson played the older divorcee who seduced River Phoenix in "A Night in the Life of Jimmy Reardon" and supported Mel Gibson, Michelle Pfeiffer and Kurt Russell in Robert Towne's "Tequila Sunrise (both 1988) and played a six-armed blue-skinned goddess who deflowers Chris Elliott in the flop "Cabin Boy" (1994). She also appeared in "Clear and Present Danger" (also 1994) and Barbet Schroeder's "Before and After" (1996), as the grieving mother of a murdered teenage girl. Magnuson has also been featured in the documentaries "Mondo New York" and "Heavy Petting" (both 1988).