She was born Melissa Chessington Leo, in New York City, NY; the daughter of Peggy Leo, a teacher, and Arnold Leo, an editor and later a fisherman. Her parents split when she was nine, with Peggy taking custody of Leo and moving to Putney, VT, before a jaunt to London, where her mother briefly supervised U.S. students in study-abroad programs. There, Leo developed an affinity for theater. When her mother returned home, she allowed her 16-year-old daughter to remain in the U.K. to pursue her studies. Leo returned to the U.S. two years later, earned her GED and enrolled at the State University of New York at Purchase. She left her junior year in search of acting work, eventually securing a role on the ABC soap "All My Children" (1970-), for which she won a Daytime Emmy nomination in 1985. She graduated to a diet of TV movies and guest shots, and along the way, entered into a volatile relationship with character actor John Heard, with whom she had a son in 1987. On an upward swing, Leo scored the female lead opposite A-list stars William Hurt and Timothy Hutton in the under-performing feature "A Time of Destiny" (1988), before spending a season on ABC's ensemble Western, "The Young Riders" (1989-1992). Leo next fell into a slate of indie films - notably two with indie auteur Henry Jaglom - including "Venice/Venice" (1992) and "Last Summer in the Hamptons" (1995), as well as a prominent role in the much ballyhooed television sequel to "Gone with the Wind" (1939), "Scarlett" (CBS, 1994). But it was in 1993, that she secured regular and trailblazing work by donning frumpy clothes and reading a male character's part to duly impress the producers of the new NBC drama, "Homicide: Life on the Street."The groundbreaking show debuted with Leo amidst a nonpareil ensemble, including Andre Braugher, Ned Beatty, Richard Belzer and Clark Johnson, which examined distinctive, memorable characters processing the tragic waste of urban America. Leo played the no-nonsense Kay Howard - initially a detective before being promoted to shift sergeant - in a way few women were seen on television. Without makeup, her red mane seemingly ever-rumpled, Leo was a standout. The harsh realism garnered the show two Emmys and a Peabody award for best television drama in its first season. But with ratings not mirroring the critical groundswell, producers eventually swapped out veteran players for younger, prettier talent and Howard was written out for the 1997-98 season. Reportedly compounding circumstances behind her departure was escalating acrimony with Heard, who had lost a lawsuit over custody of their son and, beginning in 1996, became increasingly menacing. He at one point physically assaulted Leo's boyfriend and was arrested for stalking the actress. She returned for a 2000 reunion, "Homicide: The Movie," but after a few idle years - made more so, she later said, by casting agents unable to see her as anything other than Kay Howard - she began a prolific indie film regimen.In 2003, she gave a powerful performance as the unflappable wife of an addict in "21 Grams" - earning her a nomination as Best Supporting Actress from the L.A. Film Critics Association. She raised critical eyebrows again in the Tommy Lee Jones-helmed "The Three Burials of Melquiades Estrada" (2005) and worked for Jaglom again on "Hollywood Dreams" (2006) - one of five films that year, followed by seven in 2007, including the indie favorite, "Black Irish" (2007). The following year, however, she graduated from mostly supporting roles to major indie limelight as the critically lauded lead in "Frozen River" (2008). Directed by first-time filmmaker Courtney Hunt, the film showcased Leo as a near-destitute single mother in upstate New York who teams up with a Mohawk woman to smuggle illegal aliens across the frozen St. Lawrence River. The stark, heart-rending film won the prestigious top prize at January 2008's Sundance Film Festival. Leo won the Gotham Award for Best Breakthrough Performance and netted Golden Satellite, Independent Spirit and SAG award nominations for Best Actress. Most significantly, Leo earned her first ever nod at the Academy Awards. The New York Times called Leo's performance a "magnificent portrayal of a woman of indomitable grit and not an iota of self pity " A few months later, Leo's turn as a mother coping with the abduction of her drug-addict son by a third world vice-lord in "Lullaby" (2008) won her honors at Method Fest.Leo next joined the all-star ensemble for the family dramedy "Everybody's Fine" (2009), alongside Robert De Niro, Drew Barrymore, Sam Rockwell and Kate Beckinsale. Her career shifted into high gear yet again the following year, as Leo appeared in a myriad of projects, not the least of which was the lauded cable drama "Treme" (HBO, 2010-). She joined the cast of the series as Toni Bernette, one of the denizens of a post-Katrina New Orleans ward struggling to put their lives back together. The chameleon-like actress next appeared as the emotionally stagnant wife of James Gandolfini in the exceptionally well acted melodrama "Welcome to the Rileys" (2010), directed by Jake Scott (son of Ridley). She followed with a small, but pivotal, role in the fact-based "Conviction" (2010), starring Hilary Swank as a woman who, over the course of 18 years, earned a law degree in order to exonerate her brother (Sam Rockwell), who was falsely accused of murder by a small-town cop (Leo). However, it was with her turn as Alice Ward, the indomitable mother of boxing underdog Micky Ward (Mark Wahlberg) in "The Fighter" (2010), that Leo drew the most critical praise in an already-stunning year. Directed by David O. Russell and co-starring Christian Bale, the inspiring biopic earned the much-deserving Leo Academy Award, Golden Globe and SAG Award wins for Best Supporting Actress. The following year, she found herself in awards contention again - this time at the Emmys - for her performance in the cable miniseries "Mildred Pierce" (HBO, 2011), which earned her a nod for Outstanding Supporting Actress. The rest of the year was relatively quiet for Leo, but, in 2012, she returned with a number of projects, including the little-seen character study "Francine," a supporting role in the Denzel Washington drama "Flight" and a guest spot on the cult-favorite comedy "Louie" (FX, 2010-15). She subsequently mixed appearances in big-budget fare such as the sci-fi film "Oblivion" (2013) with more intimate work in the tense drama "Prisoners" (2013), starring Hugh Jackman. During the fall of 2013, Leo hit another career high point in an Emmy win for her "Louie" appearance.