Janney, who won a scholarship to study at the Royal Academy of Dramatic Art in the summer of 1984, made her Broadway debut in the 1996 revival of Present Laughter. She won Drama Desk Awards for the 1997 Broadway revival of A View From the Bridge, and the 2009 original Broadway production of the musical 9 to 5. She also received Tony Award nominations for both. Her film roles include Primary Colors (1998), Drop Dead Gorgeous (1999), 10 Things I Hate About You (1999), American Beauty (1999), The Hours (2002), Finding Nemo (2003), Hairspray (2007), Juno (2007), The Help (2011), The Way, Way Back (2013), Mr. Peabody and Sherman (2014), Tammy (2014), Get On Up (2014), Spy (2015), Finding Dory (2016), The Girl on the Train (2016), and Tallulah (2016).
Janney was born in Boston, Massachusetts and raised in Dayton, Ohio. She is the daughter of Macy Brooks (née Putnam), a former actress and homemaker, and Jervis Spencer Janney, Jr., a real estate developer and jazz musician. She is one of three siblings.
Janney attended The Miami Valley School in Dayton, where she was named a distinguished alumna in 2004, and the Hotchkiss School in Connecticut, where she was named Alumna of the Year in 2016. She then attended Kenyon College in Gambier, Ohio. At Kenyon, she studied with Paul Newman, an alumnus of Kenyon. He directed her in a play as the inaugural event of the school's newly dedicated Bolton Theater. Newman and his wife Joanne Woodward encouraged Janney to continue acting. She then went on to train at the Neighborhood Playhouse in New York and did summer programs at the Royal Academy of Dramatic Art in London. She attended The Neighborhood Playhouse the same years as actor Dylan McDermott. She completed two year at the Neighborhood Playhouse along with Steven Rogers.
Janney's first role on television was in the short-lived black-and-white faux-1940s comedy Morton & Hayes; she appeared in two episodes of the 1991 CBS series as Eddie Hayes' wife. She then moved on to soap operas: she first played Vi Kaminski for a short time on As the World Turns, following up with a two-year role as Ginger, one of the Spaulding maids, on Guiding Light. In the spring of 1994, she appeared in the season four finale of Law & Order, as a reluctant witness against a member of the Russian mob. She was also a cast member on the radio show A Prairie Home Companion.
She has appeared in numerous films, 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 voice role in the animated movie Finding Nemo, voicing Peach, the starfish. In 2006, she had roles in the feature films Margaret and Over the Hedge.
In 1999, she was cast in the role of presidential press secretary C.J. Cregg on the television drama The West Wing, for which she eventually won four Emmy Awards. Two of the Emmys were for Primetime Emmy Award for Outstanding Supporting Actress in a Drama Series in 2000 and 2001, and two were for Primetime Emmy Award for Outstanding Lead Actress in a Drama Series in the years 2002 and 2004. She was also nominated for Outstanding Lead Actress in a Drama Series in the 2003 and 2006 Primetime Emmys.
Janney won two Screen Actors Guild awards for Best Actress in a dramatic series, in 2001 and 2002. The cast of The West Wing won the Screen Actor's Guild award for Best Ensemble in a dramatic series the same two years. Additional accolades for Janney's work in The West Wing include four Golden Globe award nominations, and a nomination in 2002 for American Film Institute's Actor of the Year. In January 2006, The West Wing's cast was also nominated by the Screen Actors Guild for Outstanding Performance by an Ensemble Cast.
In 2006, Janney was nominated for an Independent Spirit Award for her performance in the film Our Very Own. In 2007, she appeared in Juno, playing the part of Bren MacGuff, the title character's stepmother, for which she won Best Supporting Actress in the Austin Film Critics Association Awards 2007. In the same year, Janney appeared in the Golden-Globe-nominated film, Hairspray, as Prudy Pingleton, Penny's (Amanda Bynes) strict and religious mother. She appeared in the short-lived Studio 60 on the Sunset Strip in a guest appearance as herself (episode: "The Disaster Show").
Janney has remained active in theater. 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. In 2007, she participated in a workshop for a new musical of the film 9 to 5.
In late 2008, Janney joined Broadway stars Stephanie J. Block, Megan Hilty and Marc Kudisch in the new musical, 9 to 5. Based on the film of the same name, Janney starred as Violet Newstead, the super-efficient office manager played by Lily Tomlin in the original film. She was nominated for the Tony Award for Best Leading Actress in a Musical, and won the Drama Desk Award for Outstanding Actress in a Musical for the role.
In 2004, she began lending her voice to television and radio spots created by Kaiser Permanente in the health maintenance organization's broad "Thrive" media campaign, and in a radio campaign for the American Institute of Architects. In 2010, she appeared as Allison Pearson in In Plain Sight. In May 2010, she appeared in the antepenultimate episode of the ABC television series Lost as the adoptive mother of the show's two mythological opponents, Jacob and the Man in Black.
She starred in the ABC network comedy Mr. Sunshine. The series, which was created by Matthew Perry, was a midseason replacement for the 2010–11 television season.
In September 2010, it was announced that Janney would be the voice of the Aly San San spokesdroid in the Disney attraction, Star Tours: The Adventure Continues. The attraction later opened at Disney's Hollywood Studios and Disneyland. She appeared in such films as The Help (2011), Struck By Lightning (2012) and Liberal Arts (2012).
In 2014, Janney won two Emmy Awards. For her role in the series Mom, opposite Anna Faris, she won Outstanding Supporting Actress in a Comedy Series, while for her recurring role in Masters of Sex, she won Outstanding Guest Actress in a Drama Series. On the Disney Channel animated show Phineas and Ferb Janney voiced Charlene. In 2015, she appeared in the films Spy and The DUFF. In 2016, she co-starred in the fantasy film Miss Peregrine's Home for Peculiar Children, and portrayed a homicide detective in the mystery thriller The Girl on the Train. Janney will next appear in I, Tonya a biopic about Tonya Harding portraying the role of Harding's mother LaVona.
On October 11, 2016, Janney became the first woman to receive the Alumni Award of The Hotchkiss School.
On October 17, 2016, Janney received a star on the Hollywood Walk of Fame for her work in the television industry, located at 6100 Hollywood Boulevard.
In September 2015, it was reported that she is in a relationship with IATSE production coordinator Philip Joncas, 20 years her junior. By 2017, they had broken up.
In interviews related to her role on Mom, Janney discussed her brother, who fought addiction and committed suicide.
Janney previously had dreams to become an Olympic figure skater but that dream was cut short when, at age 17, she ran through a sliding glass window. The injury was serious; she cut tendons, lost arteries and a lot of blood. She has since recovered but has been left with a noticeable scar on her leg.