In the summer of 1630 he fought on the Oder against the Swedish Army led by King Gustavus Adolphus, who had occupied Pomerania.

Taken prisoner by the enemy troops, he was able to win the esteem of the king of Sweden, who did not oppose the efforts of imperial and papal diplomacy for his release.

In the mid 1630s, he left Germany and entered the staff of Cardinal-Infante Ferdinand of Austria, Governor of the Spanish Netherlands.

[citation needed] Evidence suggests that Mattei commanded his own private standing army (much smaller without additional Papal soldiers) of approximately 4000 troops.

[citation needed] When Ranuccio II Farnese refused to pay the debts assigned to him in the treaty that ended the First War of Castro, Pope Innocent X sent a force to again occupy the city.

After the end of the war, Mattei obtained permission to go to Brussels, where he became member of the staff of the Grand Condé, who had entered the service of Spain after the Fronde.

Alexander VII appointed him lieutenant general of the Papal Army with a monthly salary of 300 scudi.