George I was King of Greece from 30 March 1863 until his assassination in 1913. Originally a Danish prince, he was born in Copenhagen, and seemed destined for a career in the Royal Danish Navy. He was only 17 years old when he was elected king by the Greek National Assembly, which had deposed the unpopular Otto. His nomination was both suggested and supported by the Great Powers: the United Kingdom of Great Britain and Ireland, the Second French Empire and the Russian Empire. He married Grand Duchess Olga Constantinovna of Russia in 1867, and became the first monarch of a new Greek dynasty. Two of his sisters, Alexandra and Dagmar, married into the British and Russian royal families. King Edward VII of the United Kingdom and Emperor Alexander III of Russia were his brothers-in-law, and George V of the United Kingdom, Christian X of Denmark, Haakon VII of Norway, and Nicholas II of Russia were his nephews.

George's reign of almost 50 years (the longest in modern Greek history) was characterized by territorial gains as Greece established its place in pre-World War I Europe. Britain ceded the Ionian Islands peacefully in 1864, while Thessaly was annexed from the Ottoman Empire after the Russo-Turkish War (1877–1878). Greece was not always successful in its territorial ambitions; it was defeated in the Greco-Turkish War (1897).