Romero was born as Mathieu Rosmarin in Liège, Belgium, and, following the early death of his father was, like many children from the then Spanish Netherlands, recruited as a child to serve as choir boy at the Madrid court. Between 1586 and 1593 he was taught in Spain by his countrymen George de la Hèle and Philippe Rogier. He took the name Romero in 1594. In 1598 he was maestro de capilla at the Spanish court of Philip II of Spain and Philip III of Spain. He remained in this position till 1634. In 1609 he was ordained a priest and was private chaplain to Philippe III. He was also secretary of the Order of the Golden Fleece.
After the death of Philip, he was also chaplain to King John IV of Portugal.
Romero was one of the most appreciated composers of his time, hence “El Maestro Capitan”. His service extended over the threshold of two musical eras, the Renaissance and Baroque. Although he was out of the polyphonic school of the great Franco-Flemish school, he played an important role in the introduction of Italian “stile moderno” in Spain.