| Steve Buscemi | Enoch 'Nucky' Thompson |
| Michael Pitt | James 'Jimmy' Darmody |
| Kelly Macdonald | Margaret Schroeder |
| Michael Shannon | Nelson Van Alden |
| Shea Whigham | Elias 'Eli' Thompson |
| Aleksa Palladino | Angela Darmody
(credit only) |
| Michael Stuhlbarg | Arnold Rothstein
(credit only) |
| Stephen Graham | Al Capone |
| Vincent Piazza | Lucky Luciano |
| Paz de la Huerta | Lucy Danziger |
| Michael Kenneth Williams | Chalky White
(credit only) |
| Anthony Laciura | Eddie Kessler |
| Paul Sparks | Mickey Doyle |
| Jack Huston | Richard Harrow |
| Gretchen Mol | Gillian Darmody |
| Dabney Coleman | Commodore Louis Kaestner
(credit only) |
| Charlie Cox | Owen Sleater |
| Julianne Nicholson | Esther Randolph |
| Tony Curran | Eamonn Rohan |
| Robert Clohessy | Alderman Jim Neary |
| Kevin O'Rourke | Edward Bader |
| Anatol Yusef | Meyer Lansky |
| Nadia Alexander | Bethany Rohan |
| David K. Anderson | |
| Joseph Aniska | Agent Stan Sawicki |
| Curt Bouril | Clifford Lathrop |
| Bob Braswell | Pratt |
| Ralph Byers | Jack 'Doc' Kearns |
| Laurie Dawn | Freida Short |
| Thomas Endres | Philadelphia Enquirer Reporter |
| Shannon Garland | Nuala Rohan |
| Devin Harjes | Jack Dempsey |
| Mark Havlis | Cab Driver
(as Mark Halvis) |
| Emma Kenney | Aylesh Rohan |
| Dave Konig | Reporter 1 |
| Olan Montgomery | Irish Bartender Principal |
| Nic Novicki | Carl Heely |
| Thomas Philip O'Neill | Halsey |
| Jacqueline Pennewill | Lillian |
| Tony Rossi | Reporter 2 |
| Gary Troy | Del Grogan |
| Peter Van Wagner | Isaac Ginsburg |
| David Campbell | Eli's Deputy #3 |
| Lauren A. Kennedy | Irish Immigrant
(as Lauren Ashleigh) |
| Carolina Santos Read | Cigarette Girl in Babette's Supper Club |
| Matt Benincasa | Boxing Fan
(uncredited) |
| Danielle DeSantis | Irish Girl
(uncredited) |
| Raymond Mamrak | Laborer
(uncredited) |
| Booch O'Connell | Irish Girl
(uncredited) |
| Chloe Elaine Scharf | Irish Girl
(uncredited) |
| Meagan Weiden | Irish Girl
(uncredited) |