Royal guests included Japan's Crown Prince Naruhito, Thai Princess Maha Chakri Sirindhorn and Britain's Duke and Duchess of Gloucester. They were joined by regional political leaders.
King George, 60, had his robes designed by London tailors, and a special gold sceptre was cast for the event.
Originally scheduled for last year, the coronation was delayed as Nuku'alofa recovered from a riot sparked by a political rally in late 2006 which left eight people dead and dozens of buildings looted and burned.
Since taking power in 2006, the king has backed political reforms in the state where nearly a quarter of the 115,000 population live under the poverty line.
The king wore silk knee breeches, a medal-decked jacket, a maroon-coloured cape trimmed with white ermine fur.
His three-metre train was carried by child pages as he entered the church, followed by bearers carrying the crown, royal sceptre and ring.
He sat on a golden throne in the church as Jabez Bryce, archbishop of Polynesia, proclaimed he had been "anointed, blessed and consecrated" as king.
Bryce called on the king to rule "wisely, justly and truly".
A 21-cannon salute and the tolling of church bells marked the coronation.
He swore to maintain the constitution and laws of Tonga before being anointed on hands and forehead with oil.
The king was offered a bowl of kava, a mild narcotic drink made from the roots of a plant, to signify his sovereignty over Tonga, a country spread over 170 islands.