He was the fifth son of Theodor Hermann von Spiegel zum Desenberg und Canstein (1712-1779), the Landdrost (Lord High Steward) of the Duchy of Westphalia who had ruled that province from 1758 in the service of the Elector-Archbishop of Cologne and Duke of Westpahlia, Clemens August of Bavaria.

Descendant of an old Westphalian noble family and raised at Canstein Castle, Marsberg, Ferdinand August von Spiegel studied theology, law and economics at Fulda and Münster.

Educated in the spirit of the Enlightenment, Spiegel was in no way inclined to the status of clergy from which he only hoped for greater career opportunities, as did his elder half-brother Franz Wilhelm (1752-1815), who in 1758 succeeded his father and in 1786 became finance minister of the Electorate of Cologne.

In 1790, he accompanied the Elector-Archbishop of Cologne and Bishop of Münster, Archduke Maximilian Francis of Austria, to the coronation of the latter's brother Emperor Leopold II, in Frankfurt am Main.

Although he had been denied a ministerial position, Frederick William III of Prussia created him a Count in 1816, together with his youngest brother, Caspar Philipp (1776-1837), an Austrian ambassador.

In these years his political position changed from an advocate of a State Church to one of ecclesiastical freedom, and he became a sub-delegate of the executor of the Papal bull De salute animarum (1821).

After the king had again personally asked him to take the office, Count Spiegel was appointed archbishop of Cologne by Pope Leo XII on December 20, 1824, and ordained bishop on June 11, 1825.