Early guest-starring roles include: Buffy the Vampire Slayer (playing Janice Penshaw, the best friend of Dawn Summers), Boston Public, CSI: Miami, and Punk'd, where Ashton Kutcher and his crew members tricked her into losing someone else's dog.

In the same year, she had a recurring role alongside her eventual husband David Cross in the IFC sitcom The Increasingly Poor Decisions of Todd Margaret.

In August 2013, Tamblyn was cast as Charlie Harper's long-lost (and previously unknown) lesbian daughter, Jenny, on the sitcom Two and a Half Men, opposite Ashton Kutcher and Jon Cryer.

In 2021, Tamblyn starred opposite Diane Lane in the critically acclaimed FX television series Y: The Last Man, based on the graphic novel.

Tamblyn launched her film career playing bit parts in her father's movies: Rebellious and Johnny Mysto: Boy Wizard.

[2] Her first major film role was in 2005's The Sisterhood of the Traveling Pants as Tibby Rollins, co-starring Alexis Bledel, America Ferrera, and Blake Lively.

[15] Tamblyn appeared alongside Orlando Bloom, Colin Firth, and Patricia Clarkson in the 2010 film Main Street, a drama set in North Carolina.

[2] 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 playwright Adam Chanzit and produced by John Suits.

Tamblyn optioned the rights to the book in 2012 with the idea of producing the movie and starring as the lead character with Courtney Hunt directing.

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...

"[22] Poet Laureate Lawrence Ferlinghetti called the book, "A fine, fruitful gestation of throbbingly nascent sexuality, awakened in young new language.

[citation needed] In 2015, HarperCollins published her third collection, a hybrid of poetry and art called Dark Sparkler which explored the lives and deaths of child star actresses.

The book was a large critical success and bestseller, which features accompanying original art from Marilyn Manson, David Lynch, Marcel Dazma, Adrian Tomine, and many others.

[25] In 2019, Penguin Random House published Era of Ignition; Coming of Age in a Time of Rage and Revolution,[26] a collection of her cultural criticism and memoir essays.

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.

"[34] In March 2018, Tamblyn came under criticism for tweets about New York City's Hasidic Jewish community following an incident in Brooklyn involving her daughter.

Writing in Tablet, journalist Liel Leibovitz chided Tamblyn for "speaking so hurtfully about an entire community of underprivileged people".

[48] In October 2023, Tamblyn signed the Artists4Ceasefire open letter to Joe Biden, President of the United States, calling for a ceasefire between Israel and Gaza.

[49] In June 2021, Tamblyn wrote an essay in The New York Times expressing solidarity with Britney Spears's effort to end the conservatorship controlling her life.

Tamblyn wrote that she became financially successful when she turned 21 and starred in Joan of Arcadia after which her father became her co-manager, and her mother her business manager.

I was the one they came to for a small loan or in an emergency, the one who always picked up the check", "using money to make people happy, or fix problems, or appease my guilt", she recounted that she felt like "everybody's ATM: a bank that was, nonetheless, unconditionally loved".