Helgenberger was born in Fremont, Nebraska, to Mary Kay (née Bolte), a nurse, and Hugh Helgenberger, a meat inspector. She was raised in North Bend, Nebraska, where she graduated from North Bend Central High School. Helgenberger is of Irish and German descent and had a Roman Catholic upbringing. She has one older sister named Ann and a younger brother named Curt. Helgenberger played the French horn in her high school marching band. Until she went to college, Helgenberger aspired to be a nurse like her mother, but attended Kearney State College (now the University of Nebraska at Kearney) in Kearney, Nebraska, then attended Northwestern University's School of Speech in Evanston, Illinois, (now the School of Communication) and earned a B.S. degree in speech and drama.
Helgenberger began as a nightly weather person at KHGI-TV in Kearney,, while attending college (her name was changed by the producer to Margi McCarty). During the summer, she also worked as a deboner at her father's meatpacking plant. After portraying the role of Blanche Dubois in a university production of A Streetcar Named Desire, she developed an interest in acting.
While performing in a summer 1981 NU campus production of Shakespeare's Taming of the Shrew, where she played Kate, Helgenberger was spotted by a scout for the TV soap opera Ryan's Hope. Soon after completing college, Helgenberger landed her first professional role on the long-running ABC Daytime soap opera in March 1982, playing amateur cop Siobhan Ryan Novak DuBujak, a role previously played by Ann Gillespie. After nearly four years, Helgenberger left the show in January 1986 to pursue new opportunities.
Helgenberger guest-starred in an episode of the ABC series Spenser: For Hire, NBC's Matlock, and ABC's thirtysomething. She also played a regular role as Natalie Thayer, opposite Margot Kidder and James Read, on the six-episode drama comedy series Shell Game (1987).
She then had a role as Karen Charlene "K.C." Koloski, a heroin-addicted prostitute on the ABC war drama series China Beach from 1988 to 1991. The role earned her an Emmy Award for Outstanding Supporting Actress in a Drama Series in 1990.
In 1989, Helgenberger made her feature-film debut in a leading role as an all-night answering-service operator in one segment of the Wheat brothers’ horror anthology After Midnight. She followed it up with a role in Steven Spielberg's romantic comedy-drama Always (starring Richard Dreyfuss, Holly Hunter, and John Goodman), a modern version of the original 1943 Victor Fleming film A Guy Named Joe.
During the early to mid-1990s, Helgenberger played the love interest to Woody Harrelson's character in The Cowboy Way (1994), and had a small role as Capt. Alison Sinclair in Michael Bay's action comedy film Bad Boys (1995). She also played Dr. Laura Baker, a molecular biologist, in Roger Donaldson's science-fiction thriller, Species (1995), and reprised the role in a sequel, Species II (1998).
Helgenberger had roles in the television films Not on the Frontline and In Sickness and in Health. She played opposite Bruno Kirby in I'll Be Waiting, and as a novelist on the miniseries Stephen King's The Tommyknockers opposite Jimmy Smits. After playing a recurring role as George Clooney's love interest on NBC's medical drama ER, Helgenberger appeared as David Caruso's sex-starved widow on Showtime’s Elmore Leonard's Gold Coast. She starred with Steven Seagal in the 1997 action film Fire Down Below and portrayed the furious sibling to Steven Weber's character on the miniseries about the elusive Gulf War syndrome, Thanks of a Grateful Nation. She also starred opposite Ann-Margret in Showtime's Happy Face Murders.
In 2000, Helgenberger made a guest appearance in the Valentine's Day episode of Frasier, in which Frasier (Kelsey Grammer) finally wears down his dad Martin's resistance and gets the older man to accompany him to the opera. Actually, this invitation is but a smokescreen, so that Frasier can "accidentally" run into his newest dream girl Emily (Helgenberger).
Helgenberger co-starred in the role of Catherine Willows, a former show girl employed as a blood-spatter analyst on the CBS drama CSI: Crime Scene Investigation. Her performance as the female lead has earned her two Emmy Awards and two Golden Globe nominations. In 2005, her fellow cast members and she won the Screen Actors Guild Award for Outstanding Performance by an Ensemble in a Drama Series. When CSI first started filming, Helgenberger visited the Clark County Coroner’s Office to learn about her role, even viewing autopsies in progress. Helgenberger got the chance to act with her husband, Alan Rosenberg, when he guest-starred on CSI, season five ("Weeping Willows") and season seven ("Leaving Las Vegas").
During her time on the show, Helgenberger acted in the feature film Erin Brockovich and portrayed Patsy Ramsey on the miniseries about the mysterious murder of six-year-old beauty-pageant contestant JonBenét Ramsey in Perfect Murder, Perfect Town. She also starred as Dennis Quaid’s wife and Scarlett Johansson’s mother in writer-director Paul Weitz's romantic drama comedy In Good Company (2004).
In 2006, Helgenberger’s hometown of North Bend, Nebraska, renamed the street on which Helgenberger had her childhood home "Helgenberger Avenue".
In 2007, she was in the film Mr. Brooks with Kevin Costner in which her character's daughter is played by Danielle Panabaker, the sister of Kay Panabaker, who plays her fictional daughter on CSI.
In April 2008, Helgenberger was chosen as an endorser of the Got Milk? campaign.
In December 2009, Forbes magazine placed Helgenberger third among primetime's top-earning women, with estimated earnings of $9.5 million, after Tyra Banks and Katherine Heigl.
Helgenberger received the 2,458th star on the Hollywood Walk of Fame on January 23, 2012, for her television work.
She lent her voice as Greek goddess Hera to the straight-to-video animated film Wonder Woman.
Following William Petersen's departure from CSI, Helgenberger became one of the two highest paid actors in any of the CSI franchises, earning $375,000 per episode, the same as David Caruso, and $25,000 more per episode than Laurence Fishburne.
Helgenberger only appeared in the first 12 episodes of the 12th season of CSI, as she wanted to return to the stage. The producers said they left the door open for Helgenberger if she wanted to return. Helgenberger left the show on January 25, 2012, during a two-part episode ("Ms. Willows Regrets" and "Willows in the Wind"). The last one was the most-watched episode of the season with 14.26 million viewers.
Helgenberger also starred as Lilian Strand in the series Intelligence alongside Josh Holloway and Meghan Ory. Additionally, she briefly returned to CSI as Catherine for the 300th episode. In February 2015, she joined the third season of Under the Dome.
In 1984, Helgenberger met Alan Rosenberg, a guest actor on Ryan's Hope. The two became friends and started dating in 1986. They married in 1989 and have one son, Hugh Howard Rosenberg (b. 1990), named after Helgenberger's late father. On December 1, 2008, the couple announced that they were separating, and on March 25, 2009, she filed for divorce. Their divorce was finalized in February 2010.
As a result of Helgenberger's mother's 27-year battle against breast cancer, Helgenberger and Rosenberg became involved in the fight against breast cancer and have hosted a benefit called Marg and Alan's Celebrity Weekend annually in Omaha since 1999.Emmy Awards
1990: won – Outstanding Supporting Actress in a Drama Series – China Beach
1991: nominated – Outstanding Supporting Actress in a Drama Series – China Beach
1992: nominated – Outstanding Supporting Actress in a Drama Series – China Beach
2001: nominated – Outstanding Lead Actress in a Drama Series – CSI
2003: nominated – Outstanding Lead Actress in a Drama Series – CSI
Golden Globe Awards
1991: nominated – Best Performance by an Actress in a Supporting Role in a Series, Mini-Series or Motion Picture Made for TV – China Beach
2002: nominated – Best Performance by an Actress in a Television Series – Drama – CSI
2003: nominated – Best Performance by an Actress in a Television Series – Drama – CSI
2002: nominated – Best Performance by an Actress in a Series, Drama – CSI
People's Choice Awards
2005: won – Favorite Female TV Star – CSI
2005: won – The cast of Outstanding Performance by an Ensemble in a Drama Series – CSI
Screen Actors Guild Awards
2005: won – Outstanding Performance by an Ensemble in a Drama Series – CSI
Viewers For Quality Television Awards
1989–91: won – Best Supporting Actress in a Quality Drama Series – China Beach