Kortajarena was discovered during Fashion Week, when he met a friend attending a show. Eduardo Sayas, now his current booker, noticed him and persuaded him to try the fashion world and suggested he go to Barcelona to meet some customers. Being a minor then, Jon had to convince his parents (rather traditional) to let him travel to Paris. So he left Bilbao, his hometown, and started stepping into fashion in the French capital. Kortajarena made his debut in 2004 at the fall Emporio Armani and John Galliano shows in Milan and Paris. In the same year, he became the face of Roberto Cavalli's campaign Just Cavalli shot by Ellen Von Unwerth for the spring collection that quickly jet started his career. His runway credentials include walking for Bottega Veneta, Dolce & Gabbana, Salvatore Ferragamo, Giorgio Armani, John Galliano, and Chanel in Milan and Paris. In 2007, Kortajarena signed with GUESS? and renewed his contract again in 2008. With new campaigns on the stands at once Terry Richardson photographs Kortajarena for Tom Ford alongside Noah Mills. In the same year he was featured on the cover of the spring/summer issue of L'Officiel Hommes magazine devoting 200 pages to the Bilbao native making 2008 his busiest year. In 2009, Kortajarena appeared in Tom Ford's feature film directorial debut, A Single Man.