Guy Sebastian was born in Klang, Malaysia to a Sri Lankan and Portugese father from Malaysia, and a mother of Portuguese and Irish/Scot descent who had been born and raised in India. He moved to Adelaide, South Australia as a child. Sebastian never had any formal voice training but was influenced by artists such as Michael W. Smith and Stevie Wonder.

After giving up University studies to concentrate on music, and while working as a recording engineer and a singing teacher, he decided to enter Australian Idol establishing a fanbase and winning praise from the judges, who also made reference to his afro hairstyle, which became a focal part of his image. The final of Australian Idol in which he competed against Shannon Noll was the most popular program broadcast in Australia during 2003 and the second most popular non-sporting broadcast in Australian television history.

He is known as an accomplished artist and musician (drums, keyboard, guitar and even a little violin), and his own arrangements of the songs during the competition, together with his unique voice and innate rhythm drew many fans.

His first single "Angels Brought Me Here" reached No. 1 in 4 Asian countries and New Zealand, whilst "All I Need is You" reached no. 2 in Malaysia and No. 5 in New Zealand. He was invited to sing the single as a guest at American Idol in 2004, singing "Angels Brought Me Here" (2005 winner Carrie Underwood sang this song as well). "Kryptonite" also reached Malaysia in 2005 but management never pursued any overseas releases of any later songs.

Guy Sebastian performed at the 20th Anniversary SAFM Sky Show in Adelaide in front of well over approximately 150,000 people along with Slinkee Minx. This was one of the largest open air concerts held in Australia in 2005.

In 2005, Sebastian was appointed Ambassador for World Vision. In August of that year he visited Uganda to shoot a documentary An Idol in Africa for a Network Ten television special which was broadcast on December 3, 2005. He highlighted the issues of poverty and civil war and its effect on the population.

In 2005 he won several Nickelodeon Awards, including Favourite Aussie.

Sebastian won the publicly voted Fave Aussie gong at the 2006 Nickelodeon Australian Kids' Choice Awards in Sydney, for the second year in succession; he also performed a brand new song, "Elevator Love", earlier in the night and was presented with the Award by Bindi Irwin, daughter of the late and great Steve Irwin. The song was his second single from his third album, released in December. The album debuted at #4 on the ARIA Album charts and was certified platinum status after a few weeks.

In September 2006, he admitted to being at the receiving end of homophobic remarks, derogotary comments regarding his music career and physical harassment made by members of the public. Sebastian, who is heterosexual and has been in a long term relationship with girlfriend Julie Egan, was subjected to having his car graffitied with homosexual epithets and eggs being thrown at his house however that was some time ago. Sebastian stated that the incidents affected his work on the third album to the extent that he considered quitting being a singer altogether.

In June 2007, it was reported that Sebastian's song "Receive the Power" had been chosen as official anthem for the XXIII World Youth Day (WYD08) to be held in Sydney in 2008. The song which was written by Guy also featured the vocals of Paulini and will be released in French, Spanish and Italian. It premiered on radio in June 28 and is available on the World Youth Day website from July 1.