Peter Jeremy William Huggins was born at Berkswell Grange in Berkswell in late 1933. He was the son of a Lord Lieutenant of Warwickshire and an heir of the Cadbury chocolate family. Educated at Eton, he claimed to have been an "academic disaster", attributing his learning difficulties to dyslexia. However, he excelled at singing and was a member of the college choir. He became a drama student, but his father demanded that he change his name for the sake of the family honour.

Brett trained as an actor at the Central School of Speech and Drama in London. He made his professional acting debut at the Library Theatre in Manchester in 1954, and made his London stage debut with the Old Vic company in Troilus and Cressida 1956. In the same year he appeared on Broadway as the Duke of Aumerle in Richard II. He went on to play many classical roles on stage, including numerous Shakespearean parts in his early career with the Old Vic and later with the Royal National Theatre. Brett also played Sherlock Holmes on stage.

From the early 1960s, Brett was rarely absent from British television screens. He starred in many serials, notably as d'Artagnan in the 1966 adaptation of The Three Musketeers. A few of his appearances were in comedic roles, but usually with a classic edge, such as Captain Absolute in The Rivals. In 1973, Brett portrayed Bassanio in a televised production of William Shakespeare's The Merchant of Venice, in which Laurence Olivier portrayed Shylock and Joan Plowright Portia. (Brett, Olivier and Plowright had previously played the same roles in a Royal National Theatre production of the play.) Brett joked that, as an actor, he was rarely allowed into the 20th century and never into the present day. In reality, several of his early television appearances, in ITC series such as The Baron and The Champions saw him cast as swarthy, smooth villains very much in the present.

Although Brett's feature film appearances were relatively few, he did play Freddie Eynsford-Hill in the 1964 blockbuster film version of My Fair Lady. His singing voice was dubbed in the film, but Brett could still sing, as he later proved when he played Danilo in The Merry Widow on British television in 1968.

Brett was briefly considered by Harry Saltzman and Albert R. Broccoli for the role of James Bond in On Her Majesty's Secret Service after Sean Connery quit the series in 1967, but the role went to Australian George Lazenby instead. A second audition for the role of 007 for Live and Let Die was also unsuccessful, and Roger Moore won the coveted part.

Jeremy Brett's final, posthumous on-screen appearance was an uncredited bit part as "Artist's Father" in Moll Flanders, a 1996 Hollywood feature film starring Robin Wright Penn in the title role. The film (not to be confused with the 1996 ITV adaption starring Alex Kingston) was released nearly a year after Brett's death.

Notable in all of Jeremy Brett's roles is his precisely honed diction. Brett was born with a speech impediment, "rhotacism", that kept him from pronouncing the "R" sound correctly. Corrective surgery as a teenager, followed by years of practising, gave Brett an enviable pronunciation and enunciation. He later claimed that he practised all of his speech exercises daily, whether he was working or not.

Although he appeared in many different roles during his 40-year career, Brett is now best remembered for his performance as Sherlock Holmes in The Adventures of Sherlock Holmes, a series of Granada Television films made between 1984 and 1994. These were adapted by John Hawkesworth and other writers from the original stories by Sir Arthur Conan Doyle. Even though he reportedly feared being typecast, Brett appeared in 41 episodes of the Granada series, alongside David Burke and, latterly, Edward Hardwicke as Dr Watson.

After taking on the demanding role, Brett made few other acting appearances, and he is now widely considered to be the definitive Holmes of his era, just as Basil Rathbone was during the 1940s. Brett had previously played Doctor Watson on stage opposite Charlton Heston as Holmes in the 1980 Los Angeles production of The Crucifer of Blood, making him one of only three actors to play both Holmes and Watson professionally (the other two are Reginald Owen, as Watson in the 1932 film Sherlock Holmes and Holmes in 1933's A Study in Scarlet, and fellow Old Etonian Patrick Macnee, who played Watson first in 1976's Sherlock Holmes in New York and Holmes in 1993's The Hound of London).[citation needed]

Brett had been approached in February 1982 by Granada TV to play Holmes. The idea was to make a totally authentic and faithful adaption of the character's best cases. Eventually Brett accepted the role. He wanted to be the best Sherlock Holmes the world had ever seen. He conducted extensive research on the great detective and Sir Arthur Conan Doyle himself, and was very attentive to discrepancies between the scripts he had been given and Conan Doyle's original stories. One of Brett's dearest possessions on the set was his 77-page "Baker Street File" on everything from Holmes' mannerisms to his eating and drinking habits. Brett once explained that "some actors are becomers — they try to become their characters. When it works, the actor is like a sponge, squeezing himself dry to remove his own personality, then absorbing the character's like a liquid".