Tamzin Claire Merchant was born on March 4, 1987 in Haywards Heath, Sussex, England to Tom and Suzanne Merchant. She has one younger sister named Kate. While Tamzin was born in the UK, she spent much of her early childhood in Australia, South Africa and Dubai; however, at the age of 11, she returned to her home country to attend Windlesham House School, a boarding school for young girls and boys. There she spent her time in the school's library where she developed a love of reading, writing, and avoiding hockey. After her time at Windlesham House School, she attended Brighton College. While she twice deferred university places in order to focus on her acting career, she eventually attended and graduated Homerton College, a college of the University of Cambridge. Tamzin made her acting debut in Joe Wright's adaptation of Pride & Prejudice. Prior to her casting, Tamzin was a fan of Jane Austen's work, and wote a letter to the film's casting director explaining how she was perfect for the part of Georgiana Darcy. After meetings, she was indeed offered the role, even though she had no previous acting experience. Though, despite her lack of previous experience, Tamzin threw herself into the role, even learning how to play the piano pieces that were played on screen. Tamzin's other early credits include Margo Durrell in the 2005 BBC television film My Family and Other Animals. In 2006, she appeared in two more BBC dramas, as Sara Fox, the daughter of Alan Davies and Michelle Gomez, in the comedy The Good Housekeeping Guide, and as Probationer Eastwood in the historical drama Casualty 1906. Along with appearing in the 2008 film Princess Kaiulani.

I don’t have any acting training — I’m not a trained actor — but I think the approach is to play a character with honesty and with authenticity and a full version of themselves, rather than play a trope, if that makes sense.

Tamzin's big break came in 2009 when she appeared as the ill-fated fifth wife of Henry VIII, Katherine Howard, in Showtime's The Tudors. She made her first appearance in the third season finale and appeared throughout the fourth (and final) season. Some time after wrapping the Tudors, Tamzin was cast in the pilot for Game of Thrones as Daenerys Targaryen. However, the pilot was poorly received, and as a result the first episode was completely remade with many roles being recast (Tamzin included). This did not deter Tamzin, as she later starred in DCI Banks: 'Playing with Fire' as Miranda. And in early 2012, Merchant played the 17-year-old Rosa Bud in the BBC adaptation of Dickens' The Mystery Of Edwin Drood. Throughout the next several years Tamzin would appear in several television movies such as Red Faction: Origins and Murder on the Home Front. But it was in 2014 that Tamzin would find herself back on the small screen in WGN's Salem as Anne Hale, a young woman only beginning to discover her otherworldly powers. The series ran for three seasons before being cancelled after WGN decided to end its investment in original programming. During her time on Salem, Tamzin was able to explore a new passion - writing and directing. Over the course of three years, she was able to write and direct four short films - American Virgin, Remembering Juliet, American Pride, and American Carnage, even winning Best Screenplay at the Oxford International Film Festival, UK. Several of her productions were filmed in Shreveport and throughout the Lousiana area. Following the cancellation of Salem, Tamzin filmed several episodes of the CW series Supergirl where she played alien thief Lyra. However, her time on Supergirl was after she received a leading role in the new Amazon Original Series Carnival Row as Imogen Spurnrose. Prior to the release of the first season, Amazon greenlit a second season; however, due to the covid-19 pandemic, the season was pushed back until 2023. Carnival Row ended with its second season. Most recently, Tamzin appeared in the PBS/ITVX series Tom Jones.

Writing and directing, to me, was the logical evolution from my life as an actor: going from telling someone else's story to actually creating my own. Perhaps one day I will do all three: write, direct, and act in the same production.

Tamzin is a patron of the international development charity Build Africa, and when she is not acting or directing, she is writing. In 2019, Puffin acquired Tamzin's debut novel The Hatmakers. Set in an alternative 18th century London, The Hatmakers is about a girl who comes from a long line of magical milliners. When Cordelia’s father and his ship are lost at sea, she is determined to find him. At the same time the family’s creations, along with those of their rivals, the magical Bootmaking, Glovemaking, Cloakmaking and Watchmaking Guilds, are beginning to cause confusion and chaos. It’s up to Cordelia to find out who is behind it and why. A first book was released in 2021; it was later announced that Tamzin had signed a 3-book deal for her Cordelia Hatmakers series. Her second book in the series, The Mapmakers was released in 2022. A third, and final, book titled The Troublemakers is expected to be released in early 2024.