García-Margallo was born in Madrid. In 1960, he joined the Young Spanish Monarchists. He graduated in Law and Economics from the University of Deusto in Bilbao (1965) and subsequently received a master's degree in law (LLM) from Harvard University (1972). His great-grandfather was General Juan García y Margallo, who was killed in 1893 during the First Melillan campaign, otherwise known as the Margallo War.