He was ordained priest on 21 September 1297 in Canterbury Cathedral by Archbishop Robert Winchelsey, who gave Simon the rectory of Tunstall in Kent.

[5] Mepeham was considered to be a "man of no great ability and with scanty knowledge of ecclesiastical tradition and propriety, and the maintenance of the rights of his See caused disputes on every side.

The suffragans of Canterbury were in support of Mepeham, but his refusal to submit to the judicial process of the Church led to his excommunication by Pope John XXII in 1333.

[6] Concoreto had issued an order suspending Mepham from presiding at Divine Services on 22 January 1333 with the condition that should the Archbishop continue to refuse to resist the will of the Pope and court he was to be excommunicated 30 days later.

[3] He is buried in a tomb made of black marble located beneath the entrance arch to the Chapel of St. Anselm in Canterbury Cathedral.