• Gary Oldman as Jackson Lamb, the head of Slough House who is unkempt, flatulent, and rude with a sour wit, all disguising a tactical sharpness and firm loyalty to his agents, who he refers to affectionately as "Joes".
• Jack Lowden as River Cartwright, an up-and-coming MI5 agent abruptly shunted to Slough House after a very public training-exercise blunder.
• Kristin Scott Thomas as Diana Taverner, the power-hungry Deputy Director General of MI5 and head of operations, designated "Second Desk" until the end of series 5 when promoted to "First Desk", Director General of MI5.
• Jonathan Pryce as David Cartwright (series 4–present;^[a] special guest series 1–3), River's grandfather, a retired senior MI5 officer with a legendary reputation, often known as "The Old Bastard".
• Hugo Weaving as Frank Harkness (series 4 and 6),^[a] a former CIA operative turned ruthless mercenary. He is later revealed to be River's biological father.
• Saskia Reeves as Catherine Standish (series 5–present; co-starring series 1–4), Slough House's office administrator, a recovering alcoholic, and still enamoured with her long-deceased superior.
• Rosalind Eleazar as Louisa Guy (series 5–present;^[a] co-starring series 1–4), assigned to Slough House after a tail operation went badly wrong.
• Christopher Chung as Roddy Ho (series 5–present; co-starring series 1–4), an arrogant and obnoxious computer nerd and former hacktivist with undeniable technical skills.
• Aimee-Ffion Edwards as Shirley Dander (series 5–present; co-starring series 2–4),^[2] a physically capable agent undone by drug and anger issues.

• Sophie Okonedo as Ingrid Tearney (series 1 and 3), the Director General of MI5, often referred to as "First Desk".
• Katherine Waterston as Alison Dunn (series 3), an MI5 agent who uncovers a dark secret at the heart of the agency.
• Sope Dirisu as Sean Donovan (series 3), a former head of security at the British embassy in Istanbul.

• Dustin Demri-Burns as Min Harper (series 1–2), assigned after absent-mindedly leaving a top-secret disk on the train.
• Olivia Cooke as Sidonie "Sid" Baker (series 1), a competent, likeable MI5 agent inexplicably assigned to Slough House.
• Paul Higgins as Struan Loy (series 1), assigned after sending an inappropriate work email.
• Kadiff Kirwan as Marcus Longridge (series 2–4),^[2] who tends to gamble on the job.
• Joanna Scanlan as Moira Tregorian (series 4 and 6), one of the Queens of the Database (record keepers), banished to Slough House by Claude Whelan. She becomes Catherine's temporary replacement.
• Tom Brooke as J.K. Coe (series 4–present), the newest 'Slow Horse' and former operative from Psych Eval. He is reclusive and has PTSD.

Notable other characters appearing in more than one series include:

• Chris Reilly as Nick Duffy (series 1–3), head of MI5's internal affairs and ruthless tactical unit nicknamed the "Dogs".
• Chris Coghill as Hobbs (series 1 and 3), a member of the Dogs.
• Samuel West as Peter Judd MP (series 1–3 and 5), a ruthlessly ambitious right-wing Conservative politician who is later promoted to Home Secretary.
• Freddie Fox as James "Spider" Webb (series 1–3), a smug MI5 agent based at Regent's Park headquarters.
• Naomi Wirthner as Molly Doran (series 2–present), an ill-tempered MI5 records keeper at Regent's Park.
• Sean Gilder as "Bad" Sam Chapman (series 2–4), former Head Dog and David Cartwright's bodyguard turned private detective, whom Lamb continues to consult.
• Ruth Bradley as Emma Flyte (series 4–present), the straight-minded new head of the Dogs.
• James Callis as Claude Whelan (series 4–5), the bumbling, over-promoted new Director General of MI5 or "First Desk".

Notable other characters appearing in Series 1 include:

• Antonio Aakeel as Hassan Ahmed, Leeds University student kidnapped by the Sons of Albion.
• Paul Hilton as Robert Hobden, a disgraced and struggling journalist with ties to extremist far-right groups.
• Sam Hazeldine as Moe, the leader of the Sons of Albion, an extremist far-right group, with a secret to hide.
• Brian Vernel as Curly, a fanatical member of the Sons of Albion.
• Stephen Walters as Zeppo, a member of the Sons of Albion.
• David Walmsley as Larry, a member of the Sons of Albion.
• James Faulkner as Charles Partner (in flashbacks), a former Director General of MI5 during the Cold War, whom Standish worked for as his personal assistant.

Notable other characters appearing in Series 2 include:

• Rade Šerbedžija as Nikolai Katinsky, a former KGB agent living in exile in London after defecting at the end of the Cold War.
• Marek Vašut as Andre Chernitsky, a former KGB operative and assassin who operated during the Cold War.
• Alec Utgoff as Arkady Pashkin, a fixer for oligarch Ilya Nevsky.
• Catherine McCormack as Alex Tropper, a local resident in the small village of Upshott, the wife of Duncan and mother of Kelly.
• Adrian Rawlins as Duncan Tropper, a pub owner in Upshott, the husband of Alex and father of Kelly.
• Tamsin Topolski as Kelly Tropper, a pub barmaid in Upshott and the daughter of Alex and Duncan.
• Phil Davis as Richard Bough, aka Dickie Bow, a former MI5 officer, who was disgraced and is long since retired.

Notable other characters appearing in Series 3 include:

• Siôn Daniel Young as Douglas, an eccentric MI5 records keeper.
• Charlie Rowe as Ben, a member of Donovan's team.
• Eliot Salt as Sarah, a member of Donovan's team.
• Gavin Spokes as Sly Monteith, head of the private security firm Chieftain.
• Nick Blood as Sturges, a Chieftain operative.

Notable other characters appearing in Series 4 include:

• Kiran Sonia Sawar as Giti Rahman, an MI5 agent at Regent's Park
• Tom Wozniczka as Patrice, a mercenary working under Harkness
• Anna Wilson-Jones (voice) as Isobel Cartwright, River's mother and David's daughter.

Notable other characters appearing in Series 5 include:

• Nick Mohammed as Zafar Jaffrey, the Mayor of London who is seeking reelection^[3]
• Hiba Bennani as Tara, Roddy's apparent girlfriend^[3]
• Christopher Villiers as Dennis Gimball, a firebrand right-wing MP running for Mayor. Villiers previously had a cameo as a different far-right politician, Roger Simmonds, in series 1.
• Victoria Hamilton as Dodie Gimball, Dennis's wife and campaign manager who writes for a tabloid newspaper
• Cherrelle Skeete as Devon Welles, Flyte's deputy and fellow former Met officer turned Dog
• Monty Ben as Farouk, the leader of a Libyan terrorist cell staging attacks across London as part of a destabilisation campaign
• Fady Elsayed as Kamal, a member of Farouk's group
• Ahmed Elmusrati as Sami, a member of Farouk's group
• Abraham Popoola as Tyson Bowman, Jaffrey's chief of staff

Notable other actors set to appear in Series 6 include:^[4]

• Harry Lloyd
• Kyle Soller
• Lucian Msamati
• MyAnna Buring
• Lenny Rush

Slow Horses is based on the novel of the same name by Mick Herron, which is part of the author's Slough House series. It tells the story of a team of British intelligence agents who have all committed career-ending mistakes, and subsequently work in a dumping ground department of MI5 called Slough House.

The series was given a straight to series order by Apple TV+ in October 2019, with English writer and comedian Will Smith serving as showrunner.^[6] Each of the first five series has adapted the corresponding novel in the Slough House set. In October 2024, the show was renewed for a sixth series, which will jointly adapt the sixth and seventh books, Joe Country and Slough House.^[7] The fifth series premiered on 24 September 2025, followed by new episodes on a weekly basis until the season finale on 29 October 2025.^[8] In July 2025, ahead of the fifth series premiere, Apple TV+ renewed for a seventh series which is to be based on Bad Actors.^[9] The sixth series is set to premiere on 16 September 2026, with new episodes being released on a weekly basis until the finale on 21 October 2026.^[5]

By 2025, the series began receiving praise for its production schedule, which allows it to release seasons yearly despite being a streaming series.^[10][11] The Wall Street Journal released an op-ed regarding the show's production schedule and strategy, which includes filming seasons back-to-back, and having multiple phases of production in motion at once.^[12]

In July 2025, it was announced that Smith would depart as showrunner after the fifth series, with Gaby Chiappe taking over as head writer for the sixth series, and Ben Vanstone for the seventh.^[13] Smith described his departure as a "practical decision" in the face of difficulties managing scriptwriting for two series at a time, per the show's established schedule of simultaneously shooting one series while writing and pre-producing the next.^[14]

In October 2019, along with the series order, Gary Oldman was announced to star.^[6] The cast was rounded out in December 2020 with the additions of Olivia Cooke, Jonathan Pryce, Kristin Scott Thomas and Jack Lowden.

Since its premiere, production of Slow Horses has followed a schedule of shooting two series back to back, with the next series already having completed filming by the time the current one has released. Commentators have noted how the speed and consistency of Slow Horses' release schedule stands in contrast to the increasingly long, multi-year gaps between seasons of prestige TV shows, such as Severance and Stranger Things.^[15][16]

Filming of the first series began on 30 November 2020 in England, and continued into February 2021.^[17] In July 2021, filming continued in Stroud, Gloucestershire.^[18] It was originally intended to film earlier in 2020 but was delayed due to the COVID-19 pandemic.^[19][17] By March 2023, filming for the third series had been completed and filming for the fourth series was about to begin.^[20] In June 2025, Oldman announced that filming for the sixth series had already been completed prior to the release of the fifth, and that filming for the seventh series was expected to begin by October of that year.^[16]

Each series thus far has consisted of six episodes helmed by a single director: James Hawes directed the first series, Jeremy Lovering directed the second, Saul Metzstein directed the third and fifth, and Adam Randall directed the fourth as well as the upcoming sixth series.^[21]