Jason Isaacs was born on 6 June 1963, in Liverpool, Lancashire, in England, to Jewish parents, who later emigrated to Israel, spending his earliest childhood years in an "insular" and "closely-knit" Jewish community of Liverpudlians, of which his Eastern European great-grandparents were founder-members.

The third of four sons, Isaacs attended a Jewish school and a cheder twice a week as a young adult. When he was 11, he moved with his family to Northwest London, attending the Haberdashers' Aske's Boys' School, in Elstree, Hertsmere, Hertfordshire (Herts), where he was in the same year as film reviewer Mark Kermode. He describes his childhood as "preparation" for portraying the "unattractive", villainous characters whom he has most often played.

After completing his training as an actor, Isaacs almost immediately began appearing on the stage and on television; his film debut was in a minor role as a doctor in Mel Smith's The Tall Guy (1989). He was initially known as a TV actor in the UK, with starring roles in the ITV drama Capital City (1989) and the BBC drama Civvies (1992) and guest roles in series such as Taggart and Inspector Morse in 1992. He also played Michael Ryan in ITV1's adaptation of Martina Cole's novel Dangerous Lady, directed by Jack Woods and produced by Lavinia Warner, in 1995.

On stage he portrayed the "emotionally waffling" gay Jewish lawyer Louis Ironson in Tony Kushner's Pulitzer Prize-winning Angels in America: A Gay Fantasia on National Themes, at the Royal National Theatre, in its London première, performing the role in both parts, Part One: Millennium Approaches, in 1992, and Part Two: Perestroika, in 1993. When auditioning for that role, he told the producers, "Look, I play all these tough guys and thugs and strong, complex characters. In real life, I am a cringing, neurotic Jewish mess. Can't I for once play that on stage?"

His first Hollywood role was in the Bruce Willis blockbuster Armageddon (1998). Initially called upon to take a fairly substantial role, Isaacs was eventually cast in a much smaller capacity as a planet-saving scientist so that he could accommodate his commitment to Divorcing Jack (1998), a comedy thriller he was making with future fellow Harry Potter cast member David Thewlis.

After portraying a priest opposite Julianne Moore and Ralph Fiennes in Neil Jordan's acclaimed adaptation of Graham Greene's The End of the Affair (1999), Isaacs played the "memorable" villain, Colonel William Tavington, in Roland Emmerich's Revolutionary War fictional film epic The Patriot (2000).

Starring opposite Mel Gibson as the film's hero, and Heath Ledger as Gibson's screen son, Isaacs portrays a sadistic British army officer who kills Ledger's character, among many other soldiers. Although his work in the film earned him comparisons to Ralph Fiennes' portrayal of Nazi Amon Göth in Schindler's List (1993) and mention of a Best Supporting Actor Oscar nomination, reaching beyond being typecast as an historical villain, Isaacs chose to play a drag queen in his next project, Sweet November (2001), a romantic comedy-drama starring Charlize Theron and Keanu Reeves.

Isaacs has appeared in many other films, most notably as Lucius Malfoy in the Harry Potter series of films (2002–present). Regarding the Harry Potter books by J. K. Rowling, Isaacs has said: "I went off and read the books after the audition and I read all four books in one sitting, you know didn't wash, didn't eat, drove around with them on the steering wheel like a lunatic. I suddenly understood why my friends, who I'd thought were slightly backward, had been so addicted to these children's books. They're like crack." In "The Naked and the Dead", an article published in the San Francisco Chronicle, on 26 November 2006, Neva Chonin names the character Lucius Malfoy one of the 12 "Sexiest Men Who Were Never Alive" and Isaacs one of the 13 "Sexiest Men Who Are Real and Alive".

Prior to the making of the film, when asked whether or not he would be in Harry Potter and the Order of the Phoenix (2007), Isaacs replied, "I hope so, you'll have to ask David [producer David Heyman]. I can't bear the idea that somebody else would get to wear my Paris Hilton wig, but you never know."

Isaac also talked to J. K. Rowling on the inclusion of Lucius Malfoy in the then as-yet unpublished Harry Potter and the Deathly Hallows, so that he would have a part in the seventh and final film: "The character does not appear in the sixth book, Harry Potter and the Half-Blood Prince; but ... [Isaacs joked], 'I fell to my knees and begged ... It didn't do any good. I'm sure she doesn't need plot ideas from me. But I made my point. We'll see. Like everybody else, I'm holding my breath to July to see what's in there. I just want to bust out of prison, that's all. I don't want to stay in Azkaban most of my life.' "

However, on 12 September 2008, AceShowBiz.com revealed that Isaacs is indeed reprising his role of Lucius Malfoy as a cameo appearance in Harry Potter and the Half-Blood Prince (2009), where he is seen in a moving portrait. Afterwards, Isaacs will be reprising the role again in both parts of David Yates's film adaptations of Harry Potter and the Deathly Hallows (expected release, 2010 and 2011).

He has also appeared in Dragonheart (1996), Event Horizon (1997) , Black Hawk Down (2001), Jackie Chan's The Tuxedo (2002), and as George Darling and Captain Hook in P. J. Hogan's adaptation of Peter Pan (2003), and as the voice of Admiral Zhao in the animated Nickelodeon series Avatar: The Last Airbender (2005).

Isaacs played the leading role of Sir Mark Brydon, the British Ambassador to the United States in the BBC Four miniseries The State Within (2006), for which he was nominated for the Best Performance by an Actor in a Mini-Series or a Motion Picture Made for Television for the 65th Golden Globe Awards.

On British television, he also portrayed actor Harry H. Corbett in The Curse of Steptoe, part of "a season of new one-off dramas for BBC Four revealing the stories behind some of Britain's best loved television entertainers, and their achievements," first broadcast in March 2008.

On American television, Isaacs appeared in three episodes of The West Wing in 2004, prior to developing his most notable TV serial role, as Michael Caffee in Brotherhood (2006–present).

Between 2 February and 24 March 2007, Isaacs played Ben, opposite Lee Evans (Gus), in the critically-acclaimed 50th-anniversary production of Harold Pinter's The Dumb Waiter, at Trafalgar Studios, in London, his first theatre performance since appearing in The Force of Change (2000). He posed for photographs after his performance on 3 March 2007.