The announcement was made Tuesday by CBS Films and Lionsgate.
The film's ensemble also includes Mark Wahlberg, John Goodman, J.K. Simmons, Jimmy O. Yang, Vincent Curatola and James Colby. It is being directed by Peter Berg and produced by Wahlberg, Scott Stuber, Hutch Parker, Dylan Clark, Stephen Levinson, Dorothy Aufiero and Michael Radutzky.
Monaghan will play Carol Saunders, the wife of Wahlberg's Sergeant Tommy Saunders. Both Carol and Tommy are composite characters.
"In the aftermath of the attack, heroes appeared from every corner of Boston," Berg said in a statement, adding, "Michelle is a remarkable actress who knows the people of this city and understands their courage and strength."