Family and early life
George was born at the Yellow Palace, an 18th-century town house at 18 Amaliegade, right next to the Amalienborg Palace complex in Copenhagen. He was the second son and third child of Prince Christian of Schleswig-Holstein-Sonderburg-Glücksburg and Princess Louise of Hesse-Kassel. Until his accession in Greece, he was known as Prince William, the namesake of his grandfathers William, Duke of Schleswig-Holstein-Sonderburg-Glücksburg, and Prince William of Hesse-Kassel.
Although he was of royal blood, his family was relatively obscure and lived a comparatively normal life by royal standards. In 1853, however, George's father was designated the heir presumptive to the childless King Frederick VII of Denmark, and the family became princes and princesses of Denmark. George's siblings were Frederick (who succeeded their father as King of Denmark), Alexandra (who became wife of King Edward VII of the United Kingdom and the mother of King George V), Dagmar (who, as Empress Maria Feodorovna, was consort of Emperor Alexander III of Russia and the mother of Emperor Nicholas II), Thyra (who married Ernest Augustus, Crown Prince of Hanover) and Valdemar.
George's mother tongue was Danish, with English as a second language. He was also taught French and German. He embarked on a career in the Royal Danish Navy, and enrolled as a naval cadet along with his elder brother Frederick. While Frederick was described as "quiet and extremely well-behaved", George was "lively and full of pranks".