Newcomer Tilly Keeper is taking over the role of the teenager, who was last seen in Albert Square in 2010.
Phil (Steve McFadden) has a turbulent history with his youngest child.
Phil Mitchell is struggling with his drinking (Jack Barnes/BBC)
She was returned to her mother Lisa Fowler (Lucy Benjamin) five years ago, when Phil’s mum Peggy (Barbara Windsor) feared her son wasn’t fit to look after his daughter.
Phil went to try and get her back – but was shocked to find that she and her mother had vanished.
EastEnders bosses have decided not to reveal why Louise is back in Phil’s life, but teased: “With Phil struggling to control his drinking, her return promises to cause a stir.”
It was also revealed that Louise, now 14, has a fraught relationship with her mum and is determined to become a ‘true Mitchell’.
The Mitchell family (Kieron McCarron/BBC)
Speaking about her casting, 18-year-old Tilly commented: “I am so thrilled to be joining the cast of EastEnders, especially to be joining the Mitchell clan! It’s a dream come true and I can’t wait to get stuck into the role and see what lies in store for Louise.”
Louise will return to EastEnders early next year.