Sadler was born in Ascot, Berkshire, and took up television acting as a young girl. She was spotted as a future talent at the age of 6 by Dustin Hoffman, who had seen her in a play. After the play had finished Hoffman asked to meet the young actress. Before joining the cast of "Holby City", her best-known role had been as the character Judi Jeffreys in the children's television series "Grange Hill" from 1997 to 1999. In a tragic coincidence, her "Grange Hill" character was killed after slipping and falling out of a burning building, paralleling her real life death 4 years later. Laura also then played the part of Skirty Marm in a new BBC1 children's comedy, Belfry Witches. Skirty Marm and her friend Old Noshie (Lucy Davis) were teenage witches banished to earth from Witch Island; settling in a sleepy village where they caused mischief.