Can't really provide good answers, but I am not familiar with Popes being mummified. St. Peter is in the tombs of the Vatican, but I do not believe his remains are mummified. Most of the Popes reside in the crypt under St. Peter's. I don't know of any Popes for sure that were mummified, so obviously I can't tell you an earliest. There was talk when John XXIII was exhumed recently that his body was sort of mummified after it was exhumed to preserve it. Other then that I'm not to helpful here.
patent