| Steve Buscemi | Enoch 'Nucky' Thompson |
| Kelly Macdonald | Margaret Thompson |
| Michael Shannon | Nelson Van Alden |
| Shea Whigham | Elias 'Eli' Thompson |
| Michael Stuhlbarg | Arnold Rothstein |
| Stephen Graham | Al Capone |
| Vincent Piazza | Lucky Luciano |
| Michael Kenneth Williams | Chalky White
(credit only) |
| Anthony Laciura | Eddie Kessler |
| Paul Sparks | Mickey Doyle
(credit only) |
| Jack Huston | Richard Harrow |
| Charlie Cox | Owen Sleater |
| Bobby Cannavale | Gyp Rosetti |
| Gretchen Mol | Gillian Darmody |
| James Cromwell | Andrew W. Mellon |
| Stephen Root | Gaston Means |
| Dominic Chianese | Leander Cephas Whitlock |
| Julianne Nicholson | Esther Randolph |
| Greg Antonacci | Johnny Torrio |
| Meg Steedle | Billie Kent
(as Meg Chambers Steedle) |
| Patrick Kennedy | Dr. Douglas Mason |
| Arron Shiver | Dean O'Banion |
| Kerry O'Malley | Edwina Shearer |
| Clarke Thorell | Clifton King |
| Clea Alsip | Viola |
| John Harrington Bland | Scotty Gulliver |
| Chris Caldovino | Tonino Sandrelli |
| Joe Caniano | Jake Guzik |
| Chris Cenatiempo | Bouncer |
| Rony Clanton | Butler |
| Erin Fogel | Mary |
| Mike Houston | Ralph |
| Will Janowitz | Hymie Weiss |
| Bill Kocis | |
| Rebecca Luker | Sister Agnes |
| Randy Noojin | |
| Ned Noyes | Hank |
| Allen Lewis Rickman | George Baxter |
| Christiane Seidel | Sigrid |
| Timothy Sekk | Gilbert Longacre |
| Ryan Woodle | Phil |
| Craig Wroe | |
| Chris Angerman | Newsboy
(uncredited) |
| Anne Bergstedt | Gillian's Prostitute
(uncredited) |
| Arthur S. Brown | Sheepherder
(uncredited) |
| Ally Brunetti | Middle Class Woman 1923
(uncredited) |
| Kylie Cast | Middle Class Girl
(uncredited) |
| Carina Castagna | Middle Class Girl
(uncredited) |
| Chris J. Cullen | Bodyguard
(uncredited) |
| Steve Fogelman | Ritz Hotel Doorman
(uncredited) |
| David Itchkawitz | Upscale Citizen
(uncredited) |
| Caitlin Kerchner | Upper Class Norwegian
(uncredited) |
| Logan Kulick | Working Class Son
(uncredited) |
| Paul Jude Letersky | Farraday Iron Salesman
(uncredited) |
| Mark C. Montague | Man in White Suit
(uncredited) |
| Lynsey Leigh Price | High Class Girl
(uncredited) |
| Will Sandoval | World War I Veteran
(uncredited) |
| Anjuli Sanneman | Middle Class Woman
(uncredited) |