His television roles include Allan A Dale in three series of Robin Hood, Hotspur in Henry IV, Part I, Ashley Cowgill in Happy Valley and Bairstow in The Village.

He guest starred in the 2004 Waking the Dead episode "Fugue States" as a young man who reappears with amnesia after being abducted as a child.

Other television credits include guest spots in Midsomer Murders (2004), Blackpool (2004), Rose and Maloney (2005), another episode of The Bill (2005), The Inspector Lynley Mysteries (2006), Party Animals (2007), The Last Detective (2007) and The Whistleblowers (2007).

[7] Armstrong played Hotspur in Richard Eyre's 2012 production of Henry IV, Part I—one of four films in the BBC Two Shakespeare cycle The Hollow Crown.

Jeremy Irons played Henry IV, with Tom Hiddleston as Prince Hal and Alun Armstrong as Hotspur's father, the Earl of Northumberland.

[11] Armstrong played drug dealer-turned-kidnapper Ashley Cowgill in Sally Wainwright's 2014 crime drama Happy Valley, starring Sarah Lancashire.

[15] In 2024, Armstrong appeared in the Harlan Coben Netflix adaptation of 'Fool Me Once' as Alexander Dosman, ex-boyfriend of Claire Walker and father to their son Louis.

Performing with the National Youth Theatre, Armstrong's roles included Gerry Evans in Dancing at Lughnasa in 1998[16] and Lieutenant Stedna in They Shoot Horses, Don't They?

[22] In the DC Moore play The Empire, Armstrong starred as Gary, a Lance Corporal in Afghanistan guarding an injured prisoner who claims to be British.

Paul Taylor of The Independent wrote: "Gary ... veers between seething anger and low-key sarcasm, an oscillation superbly conveyed by excellent Joe Armstrong.

[26] Armstrong played RAF tail gunner Dusty Miller in Terence Rattigan's World War II drama Flare Path in 2011.

[29] Alfred Hickling of The Guardian remarked that "Joe Armstrong's Jean ... flips back and forth between brute arrogance and fawning servility with the casual manner of tossing a coin.

"[30] In a 2013 production of Harold Pinter's The Dumb Waiter at The Print Room, Armstrong played Gus alongside Clive Wood as Ben.