Romero was born in Huélamo, Province of Cuenca (Spain). Nothing more is known about his youth. He entered the Spanish army at the age of 16, and was stationed in Flanders. By 1547 he commanded a Spanish army unit as sergeant or captain at the Battle of Pinkie, as ally of the English in their war with Scotland known as the Rough Wooing. King Henry VIII of England was so impressed by his performance, that he made him a banneret. Romero's unit was captured during the siege of Haddington at Coldingham in the spring of 1549. He was released and stationed at Cheswick in Northumberland in May 1549. Romero fought in the Italian War of 1551–1559, where he distinguished himself in the Battle of St. Quentin (1557), for which he was made a Knight in the Order of Santiago.