Garðar Thór Cortes was born in Reykjavík, Iceland, into a family of musicians. His Icelandic father was a world-class tenor who founded the Icelandic Opera, the Reykjavík School of Singing and the Reykjavík Symphony Orchestra. His English mother, Krystyna, was a concert pianist who studied at the Royal Academy of Music in London.

At 13 he won the lead role of Nonni in Nonni and Manni, an Icelandic TV series filmed in Iceland, Norway and England and highly successful in Europe. When he turned 18, Cortes decided to pursue singing and spent four years at his father’s school in Reykjavík and then won a scholarship to the Hochschule, or University of Music and Performing Arts, Vienna, but left after six months to study privately with Professor Andrei Orlowitz in Copenhagen. Other teachers that he had the opportunity to work with included David Maxwell Anderson, Stuart Burrows, Paul Farrington, Paul Whynne Griffiths, David Jones, Kiri Te Kanawa and Robin Stapleton.

In 1999, Cortes won the lead role of Raoul, Vicomte de Chagny, in The Phantom of the Opera at Her Majesty's Theatre in London's West End. He won a scholarship to the opera course at the Royal Academy of Music and once graduated, worked across Europe, playing lead tenor roles in operas by Verdi, Rossini and Donizetti in opera houses in Germany and Scandinavia. He also sang Mendelssohn's quartets from Elijah with his sister at Carnegie Hall, New York City. Other engagements included José in Carmen Negra, Curly in Rodgers and Hammerstein's Oklahoma!, The Young Man in Dokaðu við, the Italian Tenor in Strauss's Der Rosenkavalier for the Icelandic Opera, Ferrando in Mozart's Così fan tutte for Co–Opera Ireland, Rinuccio in Puccini's Gianni Schicchi for Nordurop Opera, and a Verdi Gala at The Anvil in Basingstoke, England. His concert repertoire includes Bach's Mass in B Minor, St. Nicolas, Dvořák's Requiem, Verdi's Requiem, Handel's Messiah, Elijah, Puccini's Missa di Gloria, Rossini's Petite Messe Solennelle and Stabat Mater, and Saint-Saëns' Christmas Oratorio and Les Noces. His debut with the English Touring Opera was in Cosi Fan Tutte singing Ferrando and at the Rossini in Wildbad Festival was the role of Conte Alberto in L'occasione fa il ladro.

His self-titled debut CD in the UK shot to the top of the classical charts for three weeks in 2007 and was a close runner-up for Album of the Year at the Classical BRIT Awards. It has since achieved double platinum status.