The 72-year-old actress, who has starred in CATS Eyes and The Gentle Touch, will shock the sisters when she turns up in Walford unannounced.
Roxy (Rita Simons) and Ronnie (Samantha Womack) have never forgiven Glenda since she walked out on them and her then-husband Archie (Larry Lamb) when the girls were in their early teens.
And when she arrives in Albert Square, they will be stunned by her transformation from their nervous and quiet mother to polished, self-made woman.
Peggy Mitchell (Barbara Windsor), who knew Glenda previously, will also be amazed by how much she has changed.
Jill said: “I’m thrilled to be joining the cast of EastEnders and I am really excited to be back on British TV.”
Executive producer Diederick Santer added: “I’m so looking forward to welcoming the hugely talented Jill to EastEnders. I have a very clear memory of watching CATS Eyes every Friday night, while – to the fury of my mother – munching my way through an entire packet of Jaffa Cakes.
“Jill’s character, Maggie Forbes, was always my favourite – tough, glamorous, and exciting – perfect qualities for Glenda Mitchell.”
Jill has had a varied career as both an actress and novelist and has appeared in many television series, films and theatre productions.
Jill will appear in EastEnders from early 2010.