Nothing, however, came of the expedition, and Mordaunt returned to England in the autumn of 1679.
In June, 1680, he again sailed for Tangier with a small expedition commanded by the Earl of Plymouth.

The expedition succeeded in throwing themselves into the besieged town, and continued the defense with vigor, and Mordaunt again distinguished himself; but he soon wearied of the monotony of a long siege, and before the end of the year found opportunity to return to England, where he plunged into politics and became one of the leaders of the party formed to exclude the Duke of York from the throne.
Although a close friend of Lord Russell and Algernon Sidney he had fortunately for himself not been admitted to the fatal privilege of their private councils, and therefore escaped the fate which befell them.

He continued his friendship with them to the last, and accompanied Algernon Sidney to the scaffold.

But even while throwing himself heart and soul into politics he was continually indulging in wild freaks which rendered him the talk of the town.
On the accession of King James he made his first speech in the House of Peers against a standing army, and distinguished himself alike by the eloquence and violence of his language.

He was now under the displeasure of the court, and his profuse generosity had brought him into pecuniary trouble.