Bell was born on July 18, 1980 in Huntington Woods, Michigan, a suburb of Detroit, where she was raised. Her mother, Lorelei (née Frygier), is a registered nurse, and her father, Tom Bell, works as the television news Director for CBS Television in Sacramento, and is also an actor. Her parents divorced when she was two years old, and she has two stepsisters from her father's second marriage. She also has two half-sisters and two half-brothers from her mother's second marriage. Her mother is of Polish descent and her father has German, Scottish and Irish ancestry.

Bell is a Detroit Red Wings fan, describing the players of the team's successful late 1990's era as "gods" to her, including wearing a name tag in school declaring herself to be Mrs. Osgood. She was pictured on a ticket for a game at Joe Louis Arena in the 2000 Stanley Cup Playoffs as part of a series of fan-submitted pictures. She also said one of her first meetings with her Future husband was at a Red Wings game against the Los Angeles Kings.

In 1992, Bell went to her first audition and won a dual role as a Banana and a tree in a suburban Detroit theater's production of Raggedy Ann and Andy. Her mother had established her with an agent before Bell was 13, which allowed her to appear in newspaper advertisements for several Detroit Retailers and television commercials. She also began private acting lessons. In 1998, she appeared with an uncredited role in the locally filmed movie Polish Wedding.

Just before her freshman year of high school, Bell's parents decided to pull her from the public school system. She then attended Shrine Catholic High School in nearby Royal Oak, where she took part in the drama and music club. During her time at the school, she won the starring role in the school's 1997 production of The Wizard of Oz, as Dorothy Gale and also appeared in productions of Fiddler on the Roof (1995), Lady Be Good (1996), and Li'l Abner (1998). In 1998, the year she graduated, Bell was named the yearbook's "Best Looking Girl" by senior class vote.

In 2001, Bell left New York University to play a role as Becky Thatcher in the short-lived Broadway musical of The Adventures of Tom Sawyer. That same year, she made her credited film debut in Pootie Tang. Her one line in the film was cut and her appearance exists only as a scene shown during the credit sequence. Additionally, she auditioned for the television series Smallville for the role of Chloe Sullivan, which was eventually won by Allison Mack. In 2002, she appeared in the Broadway revival of The Crucible with Liam Neeson, Angela Bettis and Laura Linney. Bell then moved to Los Angeles, California in 2002 because of her friendship with Writers Kevin Murphy and Dan Studney, and appeared in a handful of television shows as a special guest, finding trouble gaining a recurring role in a television series. Bell had "tested like eight times and booked nothing and every show [she] tested for got picked up," including auditions for Skin and a Norm Macdonald series.

Bell also has a strong liking of Tommy Wiseau's cult film The Room (2003). She hosts parties at her house for The Room, attended cinema screenings of it, and has said that "there is a magic about that film that is indescribable".

At 24, she won the role of the title character in UPN's drama Veronica Mars, which was launched in the fall of 2004. Created by Rob Thomas, the series starred Bell as the seventeen-year-old anti-establishment high school student/detective. Bell drew on the parallels between the character of Veronica and her own life, since Bell's parents had divorced and her best friend had also died. The series earned acclaim from television critics, as did Bell's performance. Some critics asserted that her performance was overlooked, and deserved consideration for an Emmy Award.

Aside from working on Veronica Mars, Bell starred in Reefer Madness: The Movie Musical, reprising the role she played in the short-lived 2001 off-broadway musical. The musical was a spoof of the 1936 exploitation film of the same name. Reefer Madness: The Movie Musical debuted on the Showtime network on April 16, 2005. Also in April, Bell starred as Gracie in Fifty Pills, an entry for the Tribeca Film Festival. She appeared in a short independent film called The Receipt and the horror film Roman, which was directed by her The Crucible co-star Angela Bettis. Released on August 11, 2006, Pulse starred Bell as the lead Mattie. A remake of the Japanese horror film Kairo, the film grossed US$27.9 million worldwide,; however it garnered negative response from critics. Frank Scheck of The Hollywood Reporter commented, "despite the starring presence of Kristen Bell, [the] young Actress has far less interesting material to work with here than she does as [the character] `Veronica Mars.'"

In 2006 and again in 2013, Bell was selected "World's Sexiest Vegetarian" on PETA's yearly poll. She was placed No. 68 on Maxim's 2005 "Hot 100" list, No. 11 in Maxim's 2006 "Hot 100" list, and No. 46 in Maxim's 2007 "Hot 100" list in which she was stated to have "single-handedly saved The CW from becoming the worst network ever." In 2006, Maxim also placed Bell at the top of the "Fall TV's Criminally Sexy Investigators" List. In 2008, she was featured at No. 59 on Ask Men's Top 99 Women of 2008 List. Reflecting on her admitted popularity with "geeks", Bell was voted the fourth sexiest woman on TV by the staff at Wizard magazine.

In 2007, Bell ended a five-year relationship with former fiancé Kevin Mann. She later told Complex magazine that dating "makes me want to vomit. And not out of grossness—OK, a little bit out of grossness, but just nerves... I've always been a serial monogamist."

Bell supported and campaigned for Barack Obama during the 2008 United States presidential election. Along with Rashida Jones, she visited college campuses in Missouri to discuss the candidates and encourage voter registration. Bell showed support for the Writers Guild of America in the writer's strike, appearing in the picket lines in December 2007 stating, "the Writers are just looking for some fairness".

In January 2011, it was announced that Bell would be the new face of Neutrogena.

Bell stars as Jeannie van der Hooven, the female lead on the Showtime series House of Lies, which premiered on January 8, 2012. The series ended on June 12, 2016. Bell appeared in a supporting role in the science-fiction comedy Safety Not Guaranteed (2012). She starred in the drama film The Lifeguard, written and directed by Liz W. Garcia, which began filming in July 2012, and was released in August 2013. She also voiced Anna in Frozen, which was released on November 22, 2013. In 2013, for multiple episodes, Bell played Ingrid de Forest, an Eagleton City Councilwoman, on Parks and Recreation.

In late 2007, Bell began dating actor Dax Shepard, who is also from Detroit. The couple announced their engagement in January 2010. They decided to delay marriage until the state of California passed legislation legalizing same-sex marriage. They co-starred in the 2010 film When In Rome, the 2012 film Hit and Run, and the 2017 film CHiPs. After section 3 of the Defense of Marriage Act fell on June 26, 2013, Bell asked Shepard to marry her through Twitter, which he accepted. They were married at the Beverly Hills County Clerk's Office on October 17, 2013. They have two daughters: Lincoln Bell Shepard (born March, 2013) and Delta Bell Shepard (born December, 2014).

She and many of those who worked on Veronica Mars, including personal friend Ryan Hansen, are involved with the charity organization Invisible Children, Inc. The goal of the organization is to create awareness regarding the plight of Northern Ugandans who are caught in the midst of a civil war between the government and Joseph Kony's Lord's Resistance Army. Bell has also done a public Service announcement for Do Something's Healthy Living Campaign. In 2014, Bell launched a Prizeo campaign offering fans a chance to win a date with her in return for donating to Invisible Children, Inc..

In May 2016, Bell revealed that she has struggled with and received treatment for depression, anxiety and ADHD. She said, "It’s important for me to be candid about this so people in a similar situation can realize that they are not worthless and that they do have something to offer."

Also in 2017, she appeared in the biographical comedy-drama The Disaster Artist, the action comedy CHiPs and the comedy How to Be a Latin Lover. In March 2017, Bell starred in an episode of the Houzz series My Houzz, in which she surprised her sister with a major basement renovation. In November 2017, she played a housewife (with Dax Shepard as her husband), preparing for a Christmas party in Sia's music video for "Santa's Coming For Us".

In 2018, she began hosting the web series Momsplaining with Kristen Bell, with episodes airing on the Ellen DeGeneres video platform Ellentube. In the series, Bell gives new and expecting mothers tips about motherhood. The title of the series is a pun on the commonly used term "mansplaining".