The drama focuses on a working class woman who has to compete with her colleagues at a top law firm.
Montgomery has now landed the lead role of Martina, Deadline reports.
The British actress previously starred in Human Target and Entourage and also appeared in the movie Black Swan.
News of her casting in Baby Big Shot comes after Toni Trucks signed up to play Martina's friend and secretary.
Covert Affairs and Greek producer Dana Calvo is writing the Baby Big Shot script, while The West Wing producer Kevin Falls has signed up as showrunner.