Johnny Depp
James 'Whitey' Bulger

Joel Edgerton
John Connolly

Julianne Nicholson
Marianne Connolly

Dakota Johnson
Lindsey Cyr

Kevin Bacon
FBI Agent Charles McGuire

Benedict Cumberbatch
Billy Bulger

Peter Sarsgaard
Brian Halloran

Juno Temple
Deborah Hussey

Jesse Plemons
Kevin Weeks

David Harbour
John Morris

Adam Scott
FBI Agent Robert Fitzpatrick

Corey Stoll
Fred Wyshak

Bill Camp
John Callahan

Brad Carter
John McIntyre

Scott G. Anderson
Tommy King

Lonnie Farmer
DEA Agent Eric Olsen (Interrogator)

Erica McDermott
Mary Bulger

Owen Burke
Buddy Leonard

Lewis D. Wheeler
Jeremiah O'Sullivan

Jamie Donnelly
Mrs. Cody

David De Beck
Roger Wheeler

Gary Galone
State Captain

Rory Cochrane
Stephen Flemmi

James Russo
Agent Scott Gariola

Danae Nason
McGuire's Secretary

Thomas Philip O'Neill
FBI Wire Tech

Anthony Molinari
Charlie McTiernan

Declan Mulvey
Charlie's Friend

Bates Wilder
Agent James

Brian A. White
Prisoner

Tom Kemp
Father Mackey

Patrick M. Walsh
Michael Donahue

Naheem Garcia
DEA Agent

Cary 'Big Shug' Guy
Drug Dealer

Jack Neary
Barman

Arthur Hiou
Driver

Robert E. Walsh
Sr. FBI Official (voice)

W. Earl Brown
John Martorano

Jim Powers
The Mayor

Jeff Kamps
Walker

Susan Garfield
Press Photographer

Colleen Kelly
Marion Hussey (uncredited)