Allison Brooks Janney was born on November 19, 1959 in Boston, Massachusetts, and raised in Dayton, Ohio. She is the daughter of Macy Brooks Janney (née Putnam), a former actress, and Jervis Spencer Janney, Jr., a real estate developer and jazz musician. She has two brothers—Hal and Jay.

Janney made her professional stage debut in 1989 with an uncredited part in the Off-Broadway production Ladies. Following minor roles in similar productions like; Prescribed Laughter In The Emergency Café, Five Women Wearing the Same Dress and Blue Window, Janney made her Broadway debut with the 1996 revival of Noel Coward’s Present Laughter. She played Liz Essendine, the estranged wife of the lead character (played by Frank Langella). Although a minor role, her performance garnered praise and attention with The New York Times calling it “The most fully accomplished performance on the stage”. For her performance in the play, Janney won the Theatre World Award, Clarence Derwent Award for Most Promising Female, Outer Critics Circle Award for Outstanding Featured Actress in a Play and received a nomination for the Drama Desk Award for Outstanding Featured Actress in a Play.

Janney has never been married and has no children, of which she said: “I’ve never had that instinct to have kids, I’m at peace with it”. She was romantically involved with computer programmer Dennis Gagomiros. The couple began dating in 1994, and were interested in getting married but ended their relationship after seven years together in 2001. In 2002, she met actor Richard Jenik (her co-star in Our Very Own). The couple got engaged in 2004 but broke up two years later in 2006. In 2012, Janney met production manager Philip Joncas, 20 years her junior. The two met on the sets of The Way, Way Back and began dating soon after. The couple dated for five years before ending their relationship in 2017.

Janney starred in the 1998 revival of Arthur Miller’s A View from the Bridge to positive reviews. For her performance, Janney won the Drama Desk Award for Outstanding Featured Actress in a Play, Outer Critics Circle Award for Outstanding Actress in a Play, and received a nomination for the Tony Award for Best Actress in a Play.

Janney guest starred on the sitcom Frasier in a 2002 episode Three Blind Dates and appeared in the short-lived Studio 60 on the Sunset Strip in a guest appearance as herself in the episode “The Disaster Show”. In late 2009, she was chosen to play the role of Sheila Jackson in the pilot episode of Shameless, but when Showtime picked up the series, she was replaced after the pilot by Joan Cusack. In 2010, Janney 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 mid-season replacement for the 2010–11 television season.

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 September 2010, it was announced that Janney would be the voice of the Aly San San spokesdroid in the Disney attraction Star Tours – The Adventures Continue. The attraction later opened at Disney’s Hollywood Studios and Disneyland. In October 2016, Janney became the first woman to receive the Alumni Award of The Hotchkiss School and received a star on the Hollywood Walk of Fame for her work in the entertainment industry, located at 6100 Hollywood Boulevard.

Janney attended the Miami Valley School in Dayton, where she was named a distinguished alumna in 2005, and the Hotchkiss School in Connecticut, where she was named Alumna of the Year in 2016. Janney initially aspired to a career in figure skating, but her tall stature and a freak accident when she was a teenager put an end to that dream. She attended Kenyon College in Gambier, Ohio, where she majored in theatre. During her freshman year, Janney met actors Paul Newman and Joanne Woodward at a play for the inaugural event of the college’s newly built Bolton Theater, which Newman was directing. The couple encouraged her to continue acting and offered her guidance during the early days in her career. She went on to train at the Neighborhood Playhouse School of the Theatre in New York and then received a scholarship to study at the Royal Academy of Dramatic Art in the summer of 1984.

In 2007, Janney starred in Jason Reitman’s comedy-drama Juno, playing Bren MacGuff, the titular character’s stepmother. In the same year, she appeared in the musical Hairspray, and won the Austin Film Critics Association Award for Best Supporting Actress and the Critics’ Choice Movie Awards for Best Acting Ensemble. In 2010, Janney earned praise for her performance in Todd Solondz’s comedy-drama Life During Wartime.

In 2009, Janney starred in the musical 9 to 5 alongside Stephanie J. Block and Megan Hilty. Her performance garnered positive reviews, and Janney earned the Drama Desk Award for Outstanding Actress in a Musical and a nomination for the Tony Award for Best Actress in a Musical.

In 2011, Janney appeared in Tate Taylor’s period drama The Help, which won the Screen Actors Guild Award and Critics’ Choice Movie Award for Best Acting Ensemble. She also appeared in Margaret (2011), Struck by Lightning (2012), Liberal Arts (2012), Spy (2015), The DUFF (2015), Miss Peregrine’s Home for Peculiar Children (2016), Tallulah (2016), and The Girl on the Train (2016).

From 2014 to 2016, Janney guest-starred in Showtime’s period drama Masters of Sex, portraying a sexually repressed homemaker in 1950s who struggles to understand the disintegration of her marriage. Her performance received praise with Janney receiving three consecutive nominations for the Primetime Emmy Award for Outstanding Guest Actress in a Drama Series winning one in 2014, and winning the Critics’ Choice Television Award for Best Guest Performer in a Drama Series.

In 2016, Janney was honored at the White House at the event “Champions of Change”, which honored 10 accomplished individuals from across the country who were being recognized for advancing addiction prevention, treatment, and recovery. She also participated in a panel discussion with Surgeon General Vivek Murthy to talk about the portrayal of addiction and recovery in the media. In 2017, Janney donated $250,000 to Planned Parenthood. In 2020 with the outbreak of COVID-19 pandemic, Janney donated $10,000 to the Dayton Foodbank, an organisation providing the food supply to the homeless. She frequently promotes her charitable causes through her social media accounts.

In 2017, Janney starred in the black comedy I, Tonya, based on the life of figure skater Tonya Harding. Screenwriter Steven Rogers wrote the role of LaVona specifically for Janney and refused to sell his screenplay until Janney was cast in the film. Janney would consider the part as one of the most challenging of her career. Janney earned multitude of accolades for her performance in the film including the Academy Award, Golden Globe, Screen Actors Guild Award, Critics’ Choice Movie Award, Independent Spirit Award, and BAFTA Award for Best Supporting Actress.

In 2017, Janney returned to Broadway with the revival of John Guare’s Six Degrees of Separation in the role of Ouisa Kittredge. Janney received nominations for the Drama Desk Award for Outstanding Actress in a Play, Drama League Award for Distinguished Performance and won the Outer Critics Circle Award for Outstanding Actress in a Play.

In interviews related to her role on Mom, Janney discussed her brother, Hal, who fought drug addiction for years before his death by suicide. On March 4, 2018, Janney dedicated her Academy Award win to him during her acceptance speech.

Janney has campaigned for numerous issues like same-sex marriage, gay rights, women’s rights, mental health, and addiction recovery. In 2018, Janney participated in the 2018 Women’s March in Los Angeles, part of a larger national movement for women’s rights, human rights, and social justice. She supports various charities including American Heart Association, American Stroke Association, Broadway Cares/Equity Fights AIDS, and GLAAD.

Janney appeared in five movies in 2019. She had cameo appearances in the family comedy Troop Zero, the psychological thriller Ma, and Bombshell. For the last of these she received nominations at the Screen Actors Guild Award, and Critics’ Choice Movie Award Best Acting Ensemble. Janney also voiced for the role of Margaux Needler in the animated version of The Addams Family. Janney also appeared in Bad Education.