Gillan was born at Chiswick Maternity Hospital, Hounslow, London, England in a family of Scottish descent (his father was from the Govan area of Glasgow) He sang in various constellations and appeared under a variety of different pseudonyms during the early years, eg. Garth Rockett, Jess Thunder, Jess Gillan, and probably others as well.
He was the lead vocalist in the band Episode Six. After Deep Purple members Jon Lord and Ritchie Blackmore saw one of his performances with the band, he was later approached to replace Rod Evans in Deep Purple.
Gillan was a member of Deep Purple from 1969 through to 1973, appearing on such now-classic Deep Purple albums as In Rock, Fireball, Machine Head and Who Do We Think We Are. During these years, he also was the voice of Jesus on the original 1970 album recording of Jesus Christ Superstar. He was offered the lead role in the 1973 film adaptation. Ian demanded not only to be paid for his role in the movie but insisted, without the consent of his manager, that the entire band be paid because filming would conflict with a scheduled tour. The producers declined and Ian continued on in the band.
Gillan was room-mates with Deep Purple guitarist Ritchie Blackmore, and in a 2006 interview Gillan said Blackmore "turned into a weird guy and the day he walked out of the tour was the day the clouds disappeared and the day the sunshine came out and we haven't looked back since." He added that "there are certain personal issues that I have with Ritchie, which means that I will never speak to him again. Nothing I'm going to discuss publicly, but deeply personal stuff." Later, he revealed that prior to the 1993 reunion of the band, Blackmore had requested 250 thousand dollars to be deposited in his bank account in order to continue with the reunion. The other members of the band did not receive anything.
He rejoined a reunited Deep Purple in early 1984, recording the highly-acclaimed comeback album Perfect Strangers. He was sacked in 1989, but rejoined in 1992 to record the album The Battle Rages On. During the 1993 tour for this album, Ritchie Blackmore left Deep Purple for good. The rest of Deep Purple carried on, eventually replacing Blackmore with Steve Morse, and Gillan remains in the band to the present day.
After his first departure from Deep Purple, Gillan retired from performing to pursue business ventures. However, encouraged by his reception at the Butterfly Ball in 1975, he decided to resume his singing career. He formed the Ian Gillan Band. The early band sound had a distinct jazz-rock aspect which proved unpopular and was replaced by a more high powered hard rock sound as Gillan changed the lineup and shortened the group's name to Gillan. Writing the bulk of new material with keyboardist Colin Towns, the release of Mr. Universe saw Ian Gillan back in the UK charts, although the independent record company the album came out on - Acrobat - folded soon after the album was released, prompting a contract with Richard Branson's Virgin Records. Through several more lineup changes the band released a string of UK hit singles and successful albums including Glory Road, Future Shock, Double Trouble, and finally Magic.
In 1982 Ian Gillan announced the band would fold as he needed to rest his damaged vocal cords. The tone and style of his singing changed considerably when he eventually returned. His voice had a more nasal tone and this can be heard on albums he has made from 1983 to the present day. His use of multi tracking backing vocals also became highly prominent.
In 1983 he joined Black Sabbath (replacing Ronnie James Dio) for a year to record the Born Again album and tour (on which Black Sabbath played the Purple standard "Smoke On The Water" as an encore). He was largely dissatisfied with his stint in Sabbath, notably the final mix of the Born Again album (though he liked the songs and their original mixes) and its cover, which featured a demonic-looking baby. He was quoted in Kerrang! in 1984 as saying "I looked at the cover and puked." . In an interview on Part 2 of the VHS, The Black Sabbath Story (1992), he said, "I was the worst singer Black Sabbath ever had..." However, he stated in the same interview that he liked Sabbath personally: "I love Tony (Iommi), love Geezer (Butler)."
In June 2004 Gillan performed guest lead vocals on Smokescreen as part of Dean Howard - Volume One. Dean Howard (T'pau/Gillan/Repo Depo) co wrote some of the material that went towards Gillan's Dreamcatcher album.
In April 2006 Gillan released a CD/multimedia project to document his 40-year career called Gillan's Inn . Tony Iommi, Jeff Healey, Joe Satriani, Dean Howard, as well as current and former members of Deep Purple such as Jon Lord, Roger Glover, Ian Paice, Don Airey and Steve Morse are featured on this 2006 CD and DVD. The project includes a re-recorded selection of his Deep Purple, Black Sabbath and solo tracks.It was produced by Nick Blagona. In a recent interview, Gillan announced that his solo albums from the 1970s and 1980s would be re-issued late in 2006 through the Demon record company. These albums began to be released in early 2007.
Also, on September 11th, 2006, Ian Gillan promoted the Gillan's Inn tour by having local guitarists compete through local radio stations to play on stage with the band during the famous song "Smoke on the Water". The promotion was titled "Smoke This!". On the September 11th show, Lars Ulrich from the band Metallica joined Ian Gillan on stage for the song "Smoke on the Water" along with the local contest winner David Gizzarelli. Joe Satriani was scheduled to join the lineup as well, but was called to the studio for last minute revisions.
In 2006 a single called Eternity was released for the Japanese Xbox 360 game Blue Dragon, composed by Nobuo Uematsu and featuring the vocals of Gillan. That same song was reused in the fan-made, freeware RPG game Barkley, Shut Up and Jam: Gaiden for battles against VinceBorg2050 (a combination of cyborg & Vince Carter). The Eternity file in the Barkley game music folder is labeled "jesus christ the guy from deep purple sang this".
On 2 April 2007, Gillan released a DVD Highway Star: A Journey In Rock. The DVD has 6 hours of footage including documentaries and music clips.
In June 2007, he sang with the group Sed Nove and Ann Wilson in the Festival of Music in Paris.
In February 2008 Gillan released a double live album on Edel Records, "Live in Anaheim" that features Gillan and Deep Purple classic songs and several rarities. A companion DVD was released in May 2008.