Mahaffey graduated from Austin High School and attended the Bachelor of Fine Arts in University of Texas at Austin in 1975 and later made her Broadway debut in musical Rex.
She starred in the film Women of Valor in 1986 and while the characters are fictitious, it portrayed women"s roles in the Philippines during World World War World War II As of late 1980"s, Mahaffey began acting in television comedies, like Newhart, Cheers, and Seinfeld. From 1992-1993, she starred in the short-lived National Broadcasting Company sitcom The Powers That Be. She played Tracy Milford in the 1995 film National Lampoon"s Senior Trip and January Kempster in the 1997 film Jungle 2 Jungle.
In 1999, she had a recurring role on Emergency. In 2003, she starred in the film Seabiscuit.
In 2009, she appeared in seven episodes United States of Tara. Her stage credits include Eastern Standard, Talking Heads, Top Girls, and Rex.
Mahaffey played Alma Hodge in season 3 of American Broadcasting Company comedy-drama series Desperate Housewives from 2006 to 2007. Mahaffey has made guest appearances on Quantum Leap, Los Angeles She also appeared on Glee as the mother of Emma Pillsbury from 2011 to 2013.
Mahaffey co-starred as Fran Horowitz in short-lived TNT medical drama Monday Mornings in 2013.
Also in 2013, she had a recurring role on Lifetime Television comedy-drama series Devious Maids as Olivia Rice.