Janney was born in Dayton, Ohio and has two older brothers, Jay and Hal. She attended The Miami Valley School in Dayton, where she was named a distinguished alum in 2004 and the Hotchkiss School in Connecticut. She then attended Kenyon College. She went on to study drama at the Neighbourhood Playhouse in New York and the Royal Academy of Dramatic Arts in London.
Janney's first roles on television were as comedic foils on soap operas: she first played Vi Kaminski for a short time on As the World Turns, following up with a two-year stint as one of the Spaulding maids, Ginger on Guiding Light. In the spring of 1994, she appeared in the season finale of Law & Order.
Janney has appeared in a number of films with roles of various sizes, including the 1990s films American Beauty, The Object of My Affection, Big Night, The Impostors, Drop Dead Gorgeous, The Ice Storm, Primary Colors, 10 Things I Hate about You, and Private Parts, and the 2000s films Nurse Betty, The Hours, The Chumscrubber, How to Deal, Winter Solstice and a considerable role in the animated movie, Finding Nemo, voicing Peach, the starfish. In 2006, Janney had carried notable roles in the movies, Margaret and Over the Hedge, an animated comedy. Also, she took the role of Prudy Pingleton in the 2007 film Hairspray as well as that of Brenda McGuff in Juno.
In 1999, she was cast in the The West Wing, for which she has won four Emmy Awards.
Janney was honourably nominated, in 2002, for American Film Institute's Actor of the Year. A nomination of Actor/Actress of the Year from the AFI is regarded as a very high honour.
In 2006, Janney was nominated for an Independent Spirit Award for her performance in the film Our Very Own.
Janney has remained active in theatre; in 1998, she was nominated for a Tony Award for her role in Arthur Miller's A View from the Bridge Her first Broadway role was in Present Laughter opposite Frank Langella.