The light-haired, blue-eyed actor was born and raised in East London. His mother, who harbored theatrical ambitions of her own, encouraged her son's early interest in the theater. By age six, Jacobi began appearing in local library productions. A childhood bout with rheumatic fever temporarily paralyzed him, but his desire to act continued. While attending Cambridge, he joined amateur theatrical clubs and began appearing in productions, including essaying the title role in "Hamlet" (at age 18) for the England Youth Theatre at the Edinburgh Festival and the title role of "Edward II" for the university's Marlowe Society. Upon graduating, Jacobi joined the Birmingham Repertory Theatre, where he played roles ranging from Henry VIII in both Shakespeare's play and Robert Bolt's "A Man for All Seasons" and Ferdinand in "The Tempest".

Spotted by Laurence Olivier, Jacobi was invited to perform with the Chichester Festival in Shaw's "Saint Joan" in 1963. Later that year, Olivier asked him to become a founding member of the newly formed National Theatre. Jacobi made his London stage debut as Laertes in "Hamlet" at the National. The following year, he was Cassius to Olivier's "Othello" in a production that was filmed in 1965. Over the next three decades, Jacobi offered distinguished performances in such roles as Touchstone in an all-male "As You Like It" (1967, opposite Anthony Hopkins as Audrey), the title role in "Oedipus Rex" (1972), "Hamlet" (1977, 1979), "Kean" (1990), "Macbeth" (1993-94) and "Uncle Vanya" (1996). Venturing to America in 1980, he made his New York stage debut in the short-lived "The Suicide". He fared better four years later when he appeared opposite Sinead Cusack in both "Cyrano de Bergerac" and "Much Ado About Nothing". For the latter, Jacobi won the 1985 Best Actor in a Play Tony Award. A year later, he offered a tour-de-force portrayal of Alan Turing, a gay man who cracked the German Enigma code during WWII, in "Breaking the Code".

After his feature debut in "Othello", Jacobi subsequently appeared as Andrei in Laurence Olivier's "Three Sisters" (1970), as a detective's assistant in Fred Zinnemann's "The Day of the Jackal" (1973), as a printer in a pivotal sequence of "The Odessa File" (1974) and as a victim of mistaken identity in Otto Preminger's "The Human Factor" (1979). He voiced Nicodemus, a rat, in the animated feature "The Secret of NIMH" (1982). The actor has twice worked with director Christine Edzard, in "Little Dorrit" (1988, as the middle-aged bachelor in love with the title character) and "The Fool" (1990, as a 19th-century clerk who leads a double life). The actor landed one of his best screen roles, though, playing the homosexual artist Francis Bacon in the 1997 biopic "Love Is the Devil". Jacobi continued to offer scene-stealing supporting turns like his almost over-the-top art aficionado in "Up at the Villa" or his rebellious senator in "Gladiator" (both 2000). He also stood out as a valet in the all-star ensemble of Robert Altman's "Gosford Park" (2001).

In addition to his other theater and screen work, Jacobi also has forged a working relationship with actor-director Kenneth Branagh. On stage, he directed Branagh in "Hamlet" in 1988 and later that year was The Chorus in Branagh's feature remake of "Henry V" (1989). In 1991, Jacobi was mesmerizing as an antiques-collecting hypnotist in the Branagh-directed thriller "Dead Again" and he once again teamed with the younger actor-director to portray Claudius in "Hamlet" (1996).

On the small screen, Jacobi is perhaps best recalled for his brilliant, award-winning turn as the twitching, stuttering Emperor in the British miniseries "I, Claudius" (1977). He went on to give memorable performances as "Richard II" (PBS, 1979), as Adolph Hitler in the ABC miniseries "Inside the Third Reich" (1982), as the villainous Frollo to Anthony Hopkins' "The Hunchback of Notre Dame" (CBS, 1982) and as Archibald Craven in "The Secret Garden" (CBS, 1987). Jacobi won an Emmy as a mysterious stranger pretending to be a released German prisoner in "Graham Greene's 'The Tenth Man'" (CBS, 1988). In the 1990s, he has frequently lent his mellifluous voice to several projects including Ken Burns' "The Civil War" (PBS, 1990) and Burns' "Baseball" (PBS, 1994). Jacobi reprised his stage roles of "Cyrano de Bergerac" (Bravo, 1994) and Alan Turing in "Breaking the Code" (PBS, 1997) and found a new set of fans as a 12th-century sleuthing monk in "Cadfael" (PBS, 1995-99). In 2001, he garnered an Emmy nomination for his guest performance as a hammy Shakespearean actor in a memorable episode of the NBC sitcom "Frasier".