Jossa was cast in EastEnders in 2010 as Lauren Branning, taking over from Madeline Duggan who played the role from 2006 to 2010. Jossa was "excited" to join the show, saying she has always been a fan. Her first appearance was on 27 September 2010. Jossa also appears in Lauren's internet spin-off series, Lauren's Diaries, with two series in 2010 and 2011.
Jossa was nominated in the category of "Best Soap Newcomer" at the 2011 TV Choice Awards. On the 25th January Jossa won "Best Newcomer" at the National Television Awards in 2012. She was shortlisted in the category "Sexiest Female" in the "British Soap Awards 2012" but lost to Michelle Keegan (Tina McIntyre) of Coronation Street.
In 2011, Jossa started a relationship with EastEnders co-star Tony Discipline. Co-star Shona McGarty also started a relationship with Discipline's on-screen brother, Matt Lapinskas. Jossa said that she initially rejected Discipline. She also added that she knew Discipline before he joined the cast of EastEnders.