The following month he was elected Bishop of Montefiascone e Corneto and consecrated by his mother's relative, Cardinal Ulderico Carpegna.

In 1671, Altieri was named Prefect of the Sacred Consulta Propaganda Fide (a position he held until his death) and was made Governor of Tivoli.

That same year he was appointed Camerlengo of the Holy Roman Church and Crown-cardinal of Ireland (he held both positions until 1698).

Though he had established a well-regarded ecclesiastic career, his limited diplomatic experience caused problems for the Pope and for the Holy See, even after Clement's death.

He was made Archpriest of the Patriarchal Lateran Basilica in 1693 and Bishop of Porto e Santa Rufina and Sub-Dean of the College of Cardinals in January 1698.