In 1638, King Philip IV of Spain named him Envoy ad interim, to replace the inexperienced Count of Oñate as Ambassador in London.

He was soon joined in London by Virgilio Malvezzi, sent as an extraordinary ambassador by Philip IV, and by the marquis of Velada, sent by the governor of the Spanish Netherlands, the Cardinal-Infante Ferdinand of Austria.

On 21 December 1650, he was received in solemn audience by Parliament and the following month Spain became the first great power to formally recognise the Commonwealth.

[2] He played a crucial role in the acquisition of several important works of art for Philip IV and his favourite Luis Méndez de Haro in the auction organized in 1649 by the English Parliament of the collections of the king.

[5] He was withdrawn from his post following news that English forces had attacked Hispaniola as part of the Western Design, beginning the Anglo-Spanish War.

[7] Cárdenas left Britain convinced that the best hope of fomenting trouble against the Protector was by assisting the Levellers and exploiting discontent in the army rather than by means of a royalist rising.

It allied Madrid with the exiled British and Irish Royalists against their common enemies the English Commonwealth and France.