Adopting the profession of arms, he commanded ships which took part in the war between England and France in 1513 and 1514; later he served the king of Portugal against the Moors, and then he fought for his own sovereign in Ireland and in France.
After the French fleet had burnt Brighton in 1513, Henry VIII ordered Sir John Wallop to hold reprisal raids. Sir John sent his fleet and 800 men to Normandy, where he raided 21 villages, and laid in ruin several harbours, including the French Ships in Port Staples and Fraport.
In 1526 Wallop began his diplomatic career, being sent on an errand to Germany by Henry VIII; and from 1532 to 1541 he passed much of his time in Paris and elsewhere in France as the representative of the English king.
He filled several other public positions, including that of lieutenant of Calais, before January 1541, when he was suddenly arrested on a charge of treason; his offence, however, was not serious, and in the same year he was made captain of Guînes. In 1543 he led a small force to help the emperor Charles V in his invasion of France, and he remained at his post at Guînes until his death.