Born Oct. 13, 1967 in San Jose, CA, Walsh was raised in both California and Tucson, AZ. As a student, Walsh pursued acting and regional theater at the University of Arizona. Soon, she relocated to Chicago where she continued her studies with the renowned Piven Theatre Workshop and, later, the Chicago Shakespeare Repertory. Walsh's theater work in Chicago included starring roles in productions of "Flesh and Blood," "Moon under Miami,” "Troilus and Cressida," and "Born Guilty," which she also performed as a radio play on NPR. To further her acting career, Walsh relocated to New York City, where she joined the comedy troupe, Burn Manhattan, honing her comedic skills in a number of Off-Broadway plays, including "Flight Courier" and "The Four Twins."
Making a transition to the screen, Walsh was first cast in a number of small television roles on episodes of "ER" (NBC, 1994- ), "Homicide: Life on the Street" (NBC, 1993-1999), the short-lived "Swift Justice" (UPN, 1996), and "Law and Order" (NBC, 1990- ). While still performing theater in Chicago, Walsh branched out to film as well with roles in two independent films, "Normal Life" (1996), and "Henry: Portrait of a Serial Killer" (1998). She also starred as a kleptomaniac in the short film "Peppermills" (1997), which won Best Short Film at the Berlin Film Festival in 1998. Walsh's improvisational comedy training soon helped her land more prominent series regular roles on "The Mike O'Malley Show" (NBC, 1999), "The Norm Show" (ABC, 1999-2001), and HBO's "The Mind of the Married Man" (2001-2002). Walsh was then cast in a recurring role on "The Drew Carey Show" (ABC) in 2001. Donning a fat-suit, she portrayed Drew Carey's girlfriend, Nicki Fifer, a woman struggling with her weight and appearance.
Walsh went on to appear in the films "The Family Man" (2000), opposite Nicolas Cage and Tea Leoni; "Under the Tuscan Sun" (2003), as the lesbian lover of her later "Grey's Anatomy" co-star Sandra Oh; and "Kicking and Screaming" (2005), a comedy starring Will Ferrell and Robert Duvall. Walsh also had small roles in the films "After the Sunset" (2004), "One Way to Valhalla" (2005), and the family adventure film "Veritas, Prince of Truth" (2005).
Walsh scored her biggest career break to date in 2005 when she was cast in the role of Dr. Addison Shepherd on the ABC hit series "Grey's Anatomy" (2005- ). Originally a guest star slated for five episodes, Walsh's character was brought back in a recurring role as Patrick Dempsey's adulterous wife – and the bane of Meredith Grey’s existence – after her surprise appearance at the end of season one’s cliffhanger. Walsh’s layered portrayal helped viewers begrudgingly respect the talented Dr. Shepherd, despite her having broken up one of TV’s favorite couples. She had such appeal with fans, that when Shonda Rhimes decided to do a "Grey's" spinoff in 2007 – "Private Practice" – she chose Walsh's Dr. Addison as the character who could conceivably carry over.