| 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 |
| Michael Stuhlbarg | Arnold Rothstein
(credit only) |
| Stephen Graham | Al Capone |
| Vincent Piazza | Lucky Luciano |
| Paz de la Huerta | Lucy Danziger
(credit only) |
| 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 |
| Charlie Cox | Owen Sleater |
| Julianne Nicholson | Esther Randolph |
| Michael Cumpsty | Father Ed Brennan |
| Anatol Yusef | Meyer Lansky |
| Gabriel Olds | Noel Pearson |
| Joseph Aniska | Agent Sawicki |
| David Aaron Baker | Bill Fallon |
| Curt Bouril | Clifford Lathrop |
| Will Cart | Martin Sharpe |
| Pierce Cravens | Cal Widdecombe
(as William Pierce Cravens) |
| Benjamin Forster | Carruthers
(as Ben Forster) |
| Lucy Gallina | Emily Schroeder |
| Josie Gallina | Emily Schroeder |
| Joseph Harrell | Jail guard |
| Joel Hatch | Dr. Edward Holt |
| Jeb Kreager | Army recruiter |
| Heather Lind | Katy |
| Randall Newsome | Douglas Wallbridge |
| Brady Noon | Tommy Darmody |
| Connor Noon | Tommy Darmody |
| Patrick Holden O'Neill | Tom
(as Patrick O'Neill) |
| Thomas Philip O'Neill | Halsey |
| Dustin Olson | Sheriff's deputy |
| Julian Rozzell Jr. | Harlan
(as Julian Rozzel) |
| Christiane Seidel | Sigrid |
| Franklin Ojeda Smith | Deacon Lemuel Cuffy |
| Lucas Wells | LeBaron |
| Steve Triebes | WWI Recruitment Officer |
| Jonathan Weisbrod | Princeton Student |