Before Harold's acting career began she was featured on the 2011 television series Jamie's Dream School. Harold got one GCSE as she had lost a year of school due to an illness and missed out on being able to study for GCSEs. On Jamie's Dream School, she was seen as one of the brightest of the students and was rewarded by Jane Poynter and Robert Winston with the chance to stay in the biosphere which Poynter had lived in for two years. She was also given a tour of Cambridge University by David Starkey as he found her to be very intelligent and thought that it might be a good choice of university. However, she decided to go to acting school and auditioned for many soap operas.
In June 2011, EastEnders producers announced that Harold had been cast as Lola Pearce, the granddaughter of established characters Billy Mitchell and Julie Perkins. She made her first appearance in EastEnders on 12 July 2011. Harold's first major storyline involved her character's pregnancy at the age of 16. In July 2012, the character gave birth in a special live broadcast, coinciding with her screen grandfather carrying the Olympic Torch through Walford. Harold is the first EastEnders actress to portray giving birth live on air, and the second British soap actress, after Coronation Street's Jennie McAlpine.
In June 2015 it was announced that Harold would be leaving EastEnders later that year.