In the weeks leading up to the final of Australian Idol 2006, Geyer was selected by both Mark Holden and former Idol judge Ian Dickson as their favourites for winning the competition. He then secured a contract with SonyBMG after the competition ended.

Geyer was born in Johannesburg, South Africa, as the oldest of three children to parents Debbie and Keith. He has two younger sisters, Jessica and Tatum, who share his interest in martial arts. He moved to Melbourne, Australia, at age 15 leaving most of his family behind. Dean attended Melbourne High School, a select-entry school for boys, finishing his Victorian Certificate of Education from MHS in 2004, but did not pursue higher education, as he was by his own admission, "not made for school". Geyer's interest in music started early in his childhood. He learned to play the guitar, and wrote his first song entitled "Change" which was based on his move from South Africa to Australia. At his Australian Idol audition, he played a self-composed song called "Nice to Meet You".

In 2004, Dean Geyer formed a band called "Third Edge" and recorded his original music at Studio 52. Studio 52 then permitted Dean and his band to perform at the Kool Skools awards night, but they only received a nomination.

On Geyer's MySpace page, Geyer lists his musical interests as including Bon Jovi, Creed, Nickelback, Hoobastank, Goo Goo Dolls, Incubus, Lifehouse, Vertical Horizon, Switchfoot, Kelly Clarkson, Justin Timberlake, The Shaggs, Teddy Geiger, Gavin DeGraw, Fuel, Rob Thomas, Tonic, Thornley, Theory of a Deadman, The Police, Tiny Tim, U2, Matchbox 20 and Ryan Cabrera.

He has stated that his two music idols are Bon Jovi and Marc Cohn.

Geyer auditioned for the fourth series of Australian Idol in his Australian hometown of Melbourne. After singing Marc Cohn's "Walking In Memphis", Geyer played a self-composed piece called "Nice to Meet You". This song was praised by the three judges, who unanimously voted to send him through to the Top 100.

Geyer survived the culling rounds in Sydney, where the Top 100 are reduced to the Top 24. He sang "I Don't Wanna Be" by Gavin DeGraw for his elimination performance, and later performed the U2 smash hit "Beautiful Day" with Chris Murphy. They became close friends and impressed the judges with their voices and their camaraderie.

Geyer was the first contestant who was accepted by Idol into the top 100.

On 28 May, Geyer was the second contestant voted into the top 12.

Geyer's performance of Ryan Cabrera's "On the Way Down" on 15 October received a "touchdown" from judge Mark Holden. His Disco performance in the Final 8 of "Turn the Beat Around" included a front handspring by Dean off the side of the stage and a backflip at the end of his performance.