Firth would play William Weatherall Wilkins, president of Fidelity Fiduciary Bank.
Rob Marshall is directing the film, which will feature Emily Blunt and Hamilton's Lin-Manuel Miranda. Meryl Streep and Ben Wishaw are also in talks for the motion picture, which is set 25 years after the original.
Filming is expected to begin in London in early 2017. Mary Poppins Returns will hit cinemas December 25, 2018.
Producers are Marshall, John DeLuca and Marc Platt.