Tamblyn was born in Santa Monica, California. Her father, Russ Tamblyn, is an actor, dancer, and singer who starred in the 1961 film West Side Story, the 1954 film Seven Brides for Seven Brothers, and the television series Twin Peaks, and her mother, Bonnie Murray, is a singer, teacher, and artist. Her paternal grandfather, Eddie Tamblyn, was a vaudeville performer. She attended the Santa Monica Alternative School House, which, in her words, was "very unorthodox, no letter grades". At the age of ten, she played Pippi Longstocking in a school play; her father's agent, Sharon Debord, was attending as a family friend and ended up convincing her father to allow Tamblyn to go on auditions.
Tamblyn's first TV role was Emily Bowen (later known as Emily Quartermaine) on the soap opera General Hospital, a role that she played for six years (from 1995 to 2001). She also starred in "Evergreen", the pilot episode of the revived Twilight Zone series on UPN in 2002. Tamblyn became better known playing Joan Girardi, a teenage girl who receives frequent visits from God, on the CBS drama series Joan of Arcadia. Tamblyn's father made several appearances as God in the form of a dog walker on the show, which ran from 2003 to 2005.
Early guest starring roles include: Buffy the Vampire Slayer (playing Janice Penshaw, the best friend of Dawn Summers), on Boston Public, CSI: Miami, and Punk'd (where Ashton Kutcher and his crew members tricked her into losing someone else's dog). In 2007, Tamblyn starred in the CBS pilot Babylon Fields, an apocalyptic comedic drama about the undead trying to resume their former lives. The CBS network excluded the show from its fall 2007 programming lineup, since it would have competed with the network's other undead-themed drama, Moonlight.
In spring 2009, Tamblyn starred in The Unusuals, playing NYPD homicide detective Casey Shraeger in this notably acclaimed crime drama. The show was canceled after its first season. In the same year, Tamblyn had a recurring role alongside her then boyfriend (later husband) David Cross in the IFC sitcom The Increasingly Poor Decisions of Todd Margaret.
From November 2010 until April 2011, Tamblyn starred as medical student Martha M. Masters, during the seventh season of the Fox medical drama series House. She returned for the series finale in 2012.
In August 2013, Tamblyn was cast as Charlie Harper's long-lost (and previously unknown) lesbian daughter, Jenny, on the long-running sitcom Two and a Half Men, opposite Ashton Kutcher and Jon Cryer. Her first appearance was on the season 11 opener, September 26, 2013.
Tamblyn has appeared on numerous episodes of Comedy Central's Inside Amy Schumer, including the "Milk Milk Lemonade" sketch which aired in 2015. She has also guest starred on IFC's Portlandia and Comedy Bang! Bang!, as well as numerous shows on Adult Swim, including The Heart, She Holler opposite Patton Oswalt and Metalocalypse.
Tamblyn launched her film career playing bit parts in her father's movies: Rebellious and Johnny Mysto: Boy Wizard. She also appeared in 1995's Live Nude Girls. Her first major film role was in 2005's The Sisterhood of the Traveling Pants, co-starring Alexis Bledel, America Ferrera, and Blake Lively. She also starred in its 2008 sequel, The Sisterhood of the Traveling Pants 2.
Her horror film career began with the opening scene of 2002's The Ring. Tamblyn also appeared in the Japan-set The Grudge 2, a sequel to the horror hit The Grudge. The film, which also stars Sarah Michelle Gellar, was released on October 13, 2006, and debuted in the #1 spot at the North American box office. In August 2010, Tamblyn won the Bronze Leopard at the Locarno International Film Festival for her performance in the title role of Stephanie Daley. The film, which also won an award at the 2006 Sundance Film Festival, features Tamblyn as a 16-year-old who kills her baby, moments after giving birth in the bathroom of a ski resort. She was also nominated for Best Actress at the Independent Spirit Awards. The film also stars Tilda Swinton and Timothy Hutton. She also starred in Blackout in 2008.
In January 2008, Tamblyn appeared in the Hallmark film The Russell Girl, about a woman suffering from disease and mental anguish. Tamblyn appeared in the 2009 film Spring Breakdown, also featuring Amy Poehler, Rachel Dratch, and Parker Posey. Tamblyn appeared alongside Orlando Bloom, Colin Firth, and Patricia Clarkson in the 2010 film Main Street, a drama set in North Carolina. That year, she also had a role in the drama 127 Hours, with James Franco.
In 2012, Tamblyn starred alongside Wes Bentley and Vincent Piazza in the indie feature 3 Nights in the Desert directed by Gabriel Cowan, written by award-winning playwright Adam Chanzit and produced by John Suits.
In 2015, Tamblyn starred opposite Bob Odenkirk in the Netflix original film, Girlfriend's Day.
Tamblyn attended a grade school for the theatrical arts from the age of 5 to 14. She was discovered as an actress at the age of 9 when she starred in Pippi Longstocking.
In 2014 Tamblyn originated the role of Daisy Domergue for the live reading at the Ace Theater in Los Angeles of Quentin Tarantino's The Hateful Eight. She was hand picked by Tarantino, a long time friend, along with fellow cast members Samuel L. Jackson, Kurt Russell and others. Tamblyn starred in Neil LaBute's Reasons to be pretty at the Geffen Playhouse in 2014, which received critical acclaim.
Tamblyn has self-published two chapbooks of poetry, Of the Dawn and Plenty of Ships, and has participated in poetry readings at various venues, particularly in California. The Loneliest, a poem book inspired by Thelonious Monk and his music, was published in 2005 and contains haiku poetry written by Tamblyn and coupled with collages by George Herms. Only 300 copies of the book were published.
In 2005, Simon & Schuster Children's Publishing published Tamblyn's debut book of poems, written between the ages of 11 and 21, entitled Free Stallion. The School Library Journal's review states that, "Free Stallion is a compilation of poetry that amounts to a portrait of the artist as a teenager.... Many of the selections are appropriately self-absorbed but move beyond journalistic catharsis to real insight and stunning language for one so young." Poet Laureate Lawrence Ferlinghetti called the book, "A fine, fruitful gestation of throbbingly nascent sexuality, awakened in young new language." Tamblyn has toured extensively with poet Derrick C. Brown, including the now infamous Lazers of Sexcellence tours. In 2008, she was featured in the Write Bloody Publishing anthology, The Last American Valentine: Illustrated Poems to Seduce and Destroy.
Tamblyn appeared in a poetry concert film recorded on August 4, 2002 in Los Angeles, The Drums Inside Your Chest. Premiere date: April 26, 2009 at the Newport Beach Film Festival. A new collection of poetry, Bang Ditto, was published in September 2009 by San Francisco's Manic D Press. Beginning in October 2009, Tamblyn began blogging for the Poetry Foundation's blog, Harriet. Her poem "Bridgette Anderson" was one of the poems featured in Saul Williams' book Chorus, published by MTV Books in September 2011.
In 2007, she co-founded Write Now Poetry Society, dedicated to creating unique and quality poetry programming. The non-profit has a long history with The Getty Museum, curating poetry events in conjunction with art openings, such as 2011's Dark Blushing, featuring new poems commissioned by such luminaries as poets Patricia Smith and NEA fellow Jeffrey McDaniel, based on works of art by Dante Gabriel Rossetti and William Blake. Since 2011, Tamblyn has reviewed books of poetry by women for iconic feminist magazine, BUST Magazine.
In 2014, Tamblyn sold her third collection of poems, Dark Sparkler, to Harper Collins. The book explores the lives and deaths of child star actresses. Tamblyn was quoted as saying the book's subject matter was very difficult to write and took over 6 years to complete. The book also features portrait art of the women by film director and artist David Lynch and musician and artist Marilyn Manson.
She is the daughter of actor Russ Tamblyn. Her uncle is Larry Tamblyn, who is the keyboardist in the 1960s rock band The Standells, while her grandfather is actor Eddie Tamblyn.
Tamblyn and comedian/actor David Cross became engaged in August 2011, and married on October 6, 2012.
On February 21, 2017, she announced that she and Cross had recently had a daughter, Marlow Alice Cross, whose name she joked via Instagram to be Dauphinoise Petunia Brittany Scheherazade Von Funkinstein Mustard Witch RBG Cross Tamblyn-Bey jr.
She is sometimes said to be the goddaughter of actors Dean Stockwell, musician Neil Young, and the late Dennis Hopper, although in a 2009 interview with Parade, Tamblyn explained that "godfather" was "just a loose term" for Stockwell, Hopper and Young, three famous friends of her father's, who were always around the house when she was growing up, and who were big influences on her life.
In a series of tweets, which were later reiterated in an open letter published in Teen Vogue, Tamblyn said that actor James Woods tried to seduce her and a friend, as well as take them to Las Vegas, when both were sixteen and Woods was aware of the fact; the three were at a restaurant. Woods has denied the accusations, and stated that they are an attack on his political leanings.
Tamblyn's portrayal of Joan Girardi earned her both Golden Globe Award and Primetime Emmy Award nominations in 2004. She was also nominated for a Saturn Award for Best Actress in a Television Series in 2004 and 2005. Her role on House ended with her having multiple nominations for her final performance as Martha Masters.