Shannon was born on September 16, 1964 in Shaker Heights, Ohio. She found comedy at an early age and used it to help kids at a local hospital cope with their issues. After graduating from her Catholic high school, she moved to New York and earned a B.F.A. in drama from NYU's Tisch School of the Arts. Shortly after earning her degree, she moved to Los Angeles to pursue a career as an actress. While living in LA, Shannon wrote and produced the stage show "The Rob and Molly Show" with friend Rob Muir.
She found her big break with Saturday Night Live in 1995 and spent six years with the late-night live comedy show. In 2000, she earned an Emmy nomination for her work with the show. Her most well known SNL characters are "Delicious Dish" co-host Terri Rialto, self-proclaimed "Joyologist" Helen Madden, the 50-year-old dancer Sally O'Malley and Catholic school student Mary Katherine Gallagher. Shannon later starred in the 1999 film Superstar based around the Gallagher character, which was originally developed at NYU. Other television appearances include Will & Grace, Seinfeld, Sex and the City, Web Therapy, Up All Night, Glee and Kath & Kim.
In 2001, Shannon left SNL to work on her film career. Since that time, she has appeared in numerous films, including Osmosis Jones, Serendipity, Shut Up and Sing, Marie Antoinette, Talladega Nights: The Ballad of Ricky Bobby, Year of the Dog and Evan Almighty. Her more recent films include Bad Teacher, The Five-Year Engagement, Hotel Transylvania and the upcoming Scary Movie 5.
In addition to her career on screen, she is also the author of the children's book Tilly the Trickster.
Shannon currently lives with husband Fritz Chestnut and their two kids, Stella and Nolan.