Director General of the Barcelona School of Management at Pompeu Fabra University (UPF-BSM). He is also the Jean Monnet ad personam Professor for the Study of European Union Law and Government. Between 2012 and 2021, he performed different teaching, research and leaderships roles at Harvard University, working at Harvard College, the Graduate School of Arts and Sciences, the Harvard Kennedy School, and the Harvard Law School, directing the Harvard-Spanish center (the RCC), and its programs in all Harvard schools. He was a Faculty Affiliate at the Minda de Gunzburg Center for European Studies, Faculty Associate at the David Rockefeller Center for Latin American Studies, Faculty Associate at the Center for American Political Studies, and EU Law Coordinator at the Harvard Law School Institute for Global Law and Policy. He has received multiple Harvard awards for his Teaching Excellence and recognized for his extraordinary professional ability by the United States Government.   

Apart from his years at Harvard, and before joining the UPF-BSM, Martinez Sierra served in a number of roles in international and national executive boards and advisory positions in education, research and innovation. He was an advisor to several Ministers responsible for Education and Universities. He served as General Director for International Relations at the Spanish Ministry of Education and General Director of the Agency for the Internationalization of Spanish Universities. He is a former President of the European Union High Level Group on Education and Training Policies and the EU Education Committee, a member of the OCDE Education Policy Committee and former president of the Bologna Follow-up Group. He was representative for Higher Education at UNESCO and senior member of the Fulbright Commission. He is a tenured Professor on Constitutional Law at Complutense University since 2003, where was also a Jean Monnet Professor in European Constitutional Law.