| Name | Character | Episodes |
|---|---|---|
| Neal McDonough | Assistant District Attorney David McNorris | 24 |
| Jason Gedrick | Police Department Police Officer II Tom Turcotte | 24 |
| Donnie Wahlberg | Detective Joel Stevens | 24 |
| Mykelti Williamson | Police Department Detective II Bobby 'Fearless' Smith | 24 |
| Lana Parrilla | Teresa Ortiz, paramedic in season one, rookie police officer in season two | 24 |
| Gary Basaraba | Police Department Police Officer III Ray Hechler | 24 |
| Megan Ward | Kelly Stevens | 7 |
| Vanessa Williams | Detective Katherine Pierce | 6 |
| Dorian Harewood | Captain Ronald Hicks | 6 |
| Kelly Rowan | Marian McNorris | 3 |
| Kelly Hu | Rachel Durrel | 2 |
| David Proval | Paul Turcotte | 2 |
| Chase Ellison | Willie Stevens | 2 |
| Matt Craven | Dr. Michael Hirsch | 2 |
| Rick Gomez | Detective Daniel Ramos | 2 |
| Kim Murphy | Susan | 2 |
| Virginia Madsen | Erika Ashland | 1 |
| Katee Sackhoff | Holly | 1 |
| Neil Patrick Harris | Peter Corman | 1 |
| Stacy Keach | Jack 'Mac' McNorris | 1 |
| Wade Williams | Paul Barnes | 1 |
| Christian Camargo | Bradley Dawson | 1 |
| Kathleen Wilhoite | Sally McBride | 1 |
| Silas Weir Mitchell | Erik Sorenson | 1 |
| Alia Shawkat | Denise Stein | 1 |
| Chris Gartin | Jonny Rand | 1 |
| Rino Romano | Paul Turcotte in 1976 | 1 |
| David Burke | Jay Colson | 1 |
| Fredric Lehne | Reggie Flood | 1 |
| Howard Hesseman | Mel Castle | 1 |
| April Grace | Cynthia | 1 |
| Chris Mulkey | Kevin Van Horn | 1 |
| Frank John Hughes | Officer Vincent Manzani | 1 |
| Beth Grant | Landlady | 1 |
| Loren Lester | Homeowner | 1 |
| Alex Meneses | Vicki Pineda | 1 |
| Doug Hutchison | Wally Pollard | 1 |
| Ernie Grunwald | Charlie Graham | 1 |
| Christopher Gorham | Gordon Sinclair | 1 |
| Robert LaSardo | Pope | 1 |
| Bob Clendenin | Doofus | 1 |
| Kevin Dunn | Ron Berman | 1 |
| Dan Byrd | Eddie Colson | 1 |
| Kate Flannery | Tracey | 1 |
| Alimi Ballard | Young Marvin Lloyd | 1 |
| Paige Hurd | Grace Adams | 1 |
| Larry Joshua | Chet | 1 |
| Derek Hamilton | Jimmy Black | 1 |
| Gina Hecht | Doreen Stein | 1 |
| Beau Billingslea | Police Academy Commandant | 1 |
| Wendy Schaal | Darlene Beechem | 1 |
| Mark Moses | Don Schneider | 1 |
| Richard Erdman | Kevin's Dad | 1 |
| Stephen Macht | John Donadoni | 1 |
| Annika Peterson | Katrina | 1 |
| Arlen Escarpeta | Cantrel LaFontaine | 1 |
| LeVar Burton | Marvin Lloyd | 1 |
| Kim Rhodes | Julia Sloane | 1 |
| Ken Davitian | Jewelry Store Owner | 1 |
| Jennifer Sky | Vanessa Griggs / Laura | 1 |
| Tommy Hinkley | Mr. Flannery | 1 |
| Michael Dempsey | Recovering drunk | 1 |
| Jason George | Thomas Paltrow | 1 |
| Miguel A. Núñez Jr. | Freaktown | 1 |
| John Kapelos | Hugo Pinkston | 1 |
| Michael Muhney | Chris Griggs | 1 |
| Tracey Walter | Dwayne | 1 |
| Gregg Henry | Robert Colson | 1 |
| Nigel Gibbs | Alex Leonard | 1 |
| Joe Penny | Les Van Buren | 1 |
| Alexander Gould | Tom at 8 | 1 |
| Dayo Ade | Banger | 1 |
| Salvator Xuereb | John McBride | 1 |
| Joe Spano | Henry Stein | 1 |
| Cullen Douglas | Arnold Rabjohn | 1 |
| Lombardo Boyar | Eddie | 1 |
| Anthony Montgomery | Recruit Dwight Baker | 1 |
| Marquise Wilson | Friend #3 | 1 |
| Kristin Bauer | Prostitute | 1 |
| Cody Kasch | Trevor 'T.J.' Jankowski Jr. | 1 |
| Henry G. Sanders | Police Sergeant Miller | 1 |
| Ramón Franco | Luis Valdoriso | 1 |
| Mimi Paley | Rachel Stein | 1 |
| Ray Laska | Edward Nelson | 1 |
| Nathan West | Zack Berman | 1 |
| Michael Paul Chan | Lobo Chan | 1 |
| Daniel Zacapa | Carlos Ramirez | 1 |
| Erin Daniels | Officer Karen Crane | 1 |
| Robert Wisdom | Chronic (Daryl C. Norcott) | 1 |
| Patricia Wettig | Nora Jean Flannery | 1 |
| David Dunard | Irate Grandfather | 1 |
| Jessica Lundy | Vivian Colson | 1 |
| Sheeri Rappaport | Janice Edwards | 1 |
| Fran Bennett | Beatrice Rahming | 1 |
| Tyler Christopher | Holden McKay | 1 |
| Dempsey Pappion | Young Greg Haymer | 1 |
| Mark Totty | Chip Pollard | 1 |
| Bobby Hosea | Greg Haymer | 1 |
| Ali Hillis | Melanie Stone | 1 |
| Megan Henning | Emily | 1 |
| Shannon Sturges | Cherie Hechler | 1 |
| Kirk Trutner | Homeless Man | 1 |
| Garikayi Mutambirwa | Friend #2 | 1 |
| Christoph Krisea | Warden Frank Collins | 1 |
| Gregory White | Judge Harold Denby | 1 |
| Trent Cameron | Bernie Leonard | 1 |
| Rebecca Lowman | Lisa | 1 |
| Brian Sites | Taylor Gates | 1 |
| Rick Ravanello | Sean Dornan | 1 |