Trevor Clark, 44, who was said to have dwelt in a "world of fantasy", also stole more than pounds 29,000 from his village church's restoration fund.
Southwark Crown Court in south London was told that Clark, who has four children, used the money to bankroll a life of luxury. It went on expensive improvements to his five-bedroom house at Watton-at-Stone in Hertfordshire, and to send his children to private school, at a cost of pounds 21,000 a year.
Further sums went on horses for his daughters, to pay for family holidays, and to buy several cars.
Passing sentence, Judge Gerald Butler QC said the theft from the Royal Academy - where Clark was earning pounds 30,000 a year by the time of his arrest - was in "gross breach of the trust they had placed in you as one of its most senior employees".
It was "the worst" such case he had come across, went on for a number of years and was motivated "simply by greed and a desire to live at a level that your income would not support".
The systematic swindling of the15th-century parish church of St Andrew's and St Mary's, while he was its treasurer, was "particularly despicable and disreputable". "Again, your motive was greed," said the judge.
He imposed five years for stealing pounds 397,739.20 from the Royal Academy between January 1991 and December 1995, and a similar concurrent term for the theft of pounds 29,087.96 from the church restoration fund between October 1991 and February last year.Reuse content