Firth will play William Weatherall Wilkins, president of Fidelity Fiduciary Bank.
The movie also stars Meryl Streep as Poppins’ cousin. Blunt will be playing the title character, with Hamilton star Miranda portraying a street lamplighter named Jack.
The film will be directed by Rob Marshall and produced by Marshall, John DeLuca, and Marc Platt.
The screenplay will be written by Oscar nominee David Magee based on The Mary Poppins Stories by P.L. Travers. Marc Shaiman composing an all-new score and Shaiman and Scott Wittman writing original songs
Mary Poppins Returns will hit theaters on Dec. 25, 2018.
Firth has had a busy year with Genius and Bridget Jones’s Baby bowing months apart. He is also a producer on the critically acclaimed drama Loving. He can be seen next reprising his role of Harry Hart in Kingsman: The Golden Circle.
Firth is repped by CAA and Independent Talent Group.