Jason was born in his father's hometown of Liverpool on June 6, 1963. From the very beginning he had initially planned to go into law, a white-collar profession that would have fit nicely with those of his brothers, who became a doctor, lawyer, and accountant, but was swayed by acting early in the course of his law studies at Bristol University. Although he first became interested in acting in part because "it was a great way to meet girls," Jason soon found deeper meaning in the theater (in one interview he was quoted as saying "I could release myself into acting in a way that I was not released socially") and duly dropped out of Bristol to hone his skills at London's Central School of Speech and Drama.
Once in London, Isaacs began landing professional work almost immediately, appearing on the stage and on television. He made his big-screen debut in 1989 with a minor turn as a doctor in Mel Smith's The Tall Guy and that same year won a steady role on the TV series Capital City. Jason exhibited his versatility in several more TV series and on-stage in such productions as the Royal National Theatre's 1993 staging of Tony Kushner's Pulitzer Prize-winning Angels in America. He also began to find more work onscreen, receiving his first nod of Hollywood recognition in his casting in the Bruce Willis blockbuster Armageddon(1998). Initially called upon to take a fairly substantial role, Jason 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 fellow Harry Potter alumni 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, Jason got his biggest international break to date when he was picked to portray "Colonel Tavington," the resident villain of Roland Emmerich's Revolutionary War epic The Patriot. Starring opposite Mel Gibson, who (naturally) played the film's hero, Jason made an unnervingly memorable impression as a man whose pastimes included infanticide, rape, and church-burning, emerging as one of summer 2000's most indelible screen presences. Although his work in the film earned him comparisons to Ralph Fiennes' portrayal of evil Nazi "Amon Goeth" in Schindler's List and talks of a Best Supporting Actor Oscar nomination, the actor was not content to be typecast in the historical scum mold. Thus, he logically signed on to play none other than a drag queen for his next project, Sweet November (2001), a romantic comedy-drama starring Charlize Theron and Keanu Reeves.
Jason's most recent work has been with the juggernaut Harry Potter series as the sweetly vicious Lucius Malfoy.