Coleman will star as Lydia Wickham in Juliette Towhidi's PD James adaptation, joining last week's lead cast announcements of Anna Maxwell Martin (Elizabeth), Matthew Rhys (Darcy) and Matthew Goode (Wickham).
The three-part series is set six years after the events of Jane Austen's Pride and Prejudice and centres around a murder mystery plot.
Other additions to the cast include Trevor Eve as Sir Selwyn Hardcastle, Rebecca Front as Mrs Bennett, James Fleet as Mr Bennett and Penelope Keith as Lady Catherine de Burgh.
Joanna Scanlan (Mrs Reynolds), Tom Ward (Colonel Fitzwilliam), Eleanor Tomlinson (Georgiana Darcy) and James Norton (Mr Alveston) have also signed up.
Shooting starts later this month in Yorkshire and the series will be directed by Daniel Percival.
Watch Digital Spy's interview with Matthew Rhys about playing Mr Darcy: