Isaacs comes from an Eastern European Jewish family; His parents now live in Israel. He has two older and one younger brother. The first few years he grew up in a Jewish environment marked in Liverpool. When he was eleven, the family moved to the Northwest of London, where he attended a specifically targeted at boys school. In addition, Isaacs and his friends throughout the 1970s and 1980s were intensified hostility from the extreme right of the British National Front exposed. According to his own statements, his childhood not particularly pleased him, which is also one of the reasons why he embodies as an actor preferred villainous characters. Later, Isaacs studied law at Bristol University. Here he led the productions of several stage plays, in which he participated as an actor. He graduated from the Central School of Speech and Drama in London.
He had his first small role in the film The Tall Guy in 1989. Subsequently, he was engaged for a longer period for the British television series " Capital City" and also acted in several other British television series with. In 1997, he starred in Event Horizon - On the edge of the universe for the first time in a Hollywood film. A year later he was in the blockbuster Armageddon - to see The Last Judgment as NASA scientists. In 2000, he starred in Roland Emmerich's The Patriot, the role of Colonel Tavington, the opponent of lead actor Mel Gibson.
Among his most famous roles include that of Lucius Malfoy, he embodied since Harry Potter and the Chamber of Secrets (2002) in all the following parts of the film series, as well as that of Captain Hook from the remake of Peter Pan in 2003. 2006 played Isaacs on the BBC miniseries the State Within the main role for which he received a Golden Globe nomination. In the American series Brotherhood, he embodied the same year also the main role. As a theater actor, he was active in 1992 and appeared in several stage plays at London's Royal National Theatre, in which he always embodied a gay, Jewish character. Since 2007 he was again amplified in the theater industry and played along with comedian Lee Evans in a play called The Dumb Waiter at Trafalgar Studios in London.
Isaacs is since 1988 a relationship with the BBC documentary filmmaker Emma Hewitt. The two got married in 2001 and have two daughters.
In German-speaking several times been dubbed his voice by Hubertus Bengsch, Tom Vogt, and by Wolfgang Müller (especially in the Harry Potter films ).