Van Mierlo was named the Bishop of Haarlem and the Abbot of Egmond Abbey in March 1570 by Pope Pius V. He was welcomed to that city in early 1571, where he was consecrated a bishop on February 11 of that year in the original Cathedral of St. Bavo by Franciscus van der Velde, the Bishop of Antwerp. The following year, however, on December 4, 1572, the cathedral suffered the iconoclastic destruction of the "Beeldenstorm" and temporarily became the scene of Calvinist services, which lasted until the end of the Siege of Haarlem. Van Mierlo had to re-sanctify the church in 1573. In Veere, in 1577, Van Mierlo set his seal to the "Satisfactie van Haarlem" wherein he promised to swear allegiance to Willem the Silent rather than Philip II of Spain, on the condition that the Catholics would keep the same rights as Protestants.