Mier-Romeu's main experience is in general business or commercial litigation, administrative agencies, and corporate law. He began his legal career at the litigation division of Puerto Rico's oldest and then-largest law firm, and subsequently worked at a boutique law firm specializing in insurance law. Before becoming an attorney, Mier-Romeu worked as a journalist at Puerto Rico's leading newspaper, El Nuevo Día, covering business, economics, and government affairs. Besides his law degree from the University of Puerto Rico, Mier-Romeu majored in economics at Colby College and studied mathematics at the graduate level in the University of Puerto Rico, where he also taught the subject. Mier-Romeu's first job after college was as a financial analyst and registered representative at Merrill Lynch Puerto Rico's investment banking division.