Theobold was educated at Outwood Grange Academy before attending the Guildhall School of Music and Drama. She initially auditioned for the role of Ivy over Christmas 2011, during her final year. She was allowed to finish her final year of drama school early to begin filming the series at Easter. Theobold characterised Ivy as "ambitious" and "a dreamer". She returned for the fourth series of the show in 2013. Her work on the series resulted in her being a co-recipient of the Screen Actors Guild Award for Outstanding Performance by an Ensemble in a Drama Series.

In 2014, Theobold made a guest appearance on the TV sitcom Lovesick (formerly known as Scrotal Recall). In 2015, she appeared in two episodes of Last Tango in Halifax as Holly, and an episode of Call the Midwife. She starred as Sarah, a housekeeper at a stately home and one of the main characters, in the third series of The Syndicate which aired in spring 2015. She is also the voice Actress for Tracer in the video games Overwatch and Heroes of the Storm and starred in the E4/Netflix original series Crazyhead alongside Susan Wokoma. In 2016, it was announced that Theobold would star in Absentia along with Stana Katic and Patrick Heusinger.