Cohan was born in Cherry Hill, New Jersey and raised there until age 13, when her family moved to her mother's native United Kingdom, settling in Surrey. Her father is American of Irish and Norwegian descent, while her mother is Scottish, and returned to the U.S. when she was around 17. In addition to Cherry Hill, Cohan lived for one year in the state of Georgia before her family moved to the UK. Lauren's surname was legally changed to that of her stepfather, Cohan, and her mother converted to Judaism when she remarried. Cohan was also converted to Judaism at age five and had a bat mitzvah ceremony. She has four younger siblings and one step-sister. Cohan graduated from King Alfred's College, Winchester with a Bachelor of Arts degree in drama and English literature before touring with a theatre company she had co-founded at the university.
After her film debut in Casanova as Sister Beatrice, she starred in the 2006 sequel to Van Wilder, ' as Charlotte Higginson. Her next role was in the 2008 film Float. In February 2010, she was cast in Death Race 2, and was a lead in the 2016 supernatural-horror The Boy. In 2016, she appeared as Martha Wayne in ', and played Leila Steinberg in the biographical drama ''All Eyez on Me |All Eyez on Me
In 2007, Cohan gained her first major supporting role – partially also described as a season lead – when she was cast for season three of Supernatural as Bela Talbot, a thief who procures valuable supernatural objects and sells them to very rich, very powerful people in the supernatural world. The character appeared in six episodes of the series, her last in the penultimate episode of season three. Cohan played the recurring character Rose, a 560-year-old vampire, in The Vampire Diaries. In 2011, she joined the television series Chuck in a recurring role playing Vivian McArthur Volkoff, a "charming, sophisticated socialite" from the United Kingdom, and the daughter of primary villain, Alexei Volkoff. She also guest starred on shows such as Modern Family, , Cold Case, Life, and Archer. Cohan starred in a pilot for The CW called Heavenly in which she played the lead character, an attorney who teams up with an angel; however, it was not picked up as a series. In April 2011, Cohan was cast in her most notable role as Maggie Greene on The Walking Dead, an AMChorror drama series based on the comic book series of the same name. She first appeared in season two as a recurring character, but she became a series regular at the start of season three. The drama was the highest-rated series in cable television history in 2015. At the conclusion of the eighth season of The Walking Dead, Cohan completed her contract obligations as a series regular. It was later reported Cohan had not reached an agreement to sign on for season 9 as a main cast member due to a pay dispute, as she demanded an enhanced salary closer to the salaries of her male co-stars Andrew Lincoln and Norman Reedus. AMC refused, and Cohan began making herself available for TV pilots. Cohan eventually reached an agreement to appear in five episodes in the first half of season nine of The Walking Dead and departed the show in 2018. In early 2018, in the midst of her contract negotiations with AMC regarding The Walking Dead, Cohan was cast as CIA operative Francesca "Frankie" Trowbridge in the ABC spy drama Whiskey Cavalier. Whiskey Cavalier was picked up to series in May 2018 and premiered on February 24, 2019. ABC did not renew the show for a second season. In October 2019, it was reported that Cohan would return to The Walking Dead as a series regular in the show's eleventh season. Furthermore, showrunner Angela Kang indicated that Cohan might appear during the show's then-current season as well.
Cohan splits her time and work between London and Los Angeles. She is known for her distinctive Mid-Atlantic accent. Cohan began dating Christian Carino, her agent at that time, in 2014, but they split in 2017.