Diogo is currently a lecturer at the Portuguese Military Academy and was, until 2023,  a lecturer at the Faculty of Law of the University of Porto, where he concluded his PhD in 2016. He holds a Master’s in Law from the Faculty of Law of the University of Coimbra. He was a member of the Commission for the Reform of the Personal Income Tax (IRS – Imposto sobre as Pessoas Singulares), in 2014, and has been Tax Arbitrator at the Centre for Administrative Arbitration (CAAD – Centro de Arbitragem Administrativa) since the same year.

He was a Member of the European Parliament during the VII Legislature, from 2009 to 2014. He was an effective member of the Committee for Economic and Monetary Issues and of the Special Committee for the Financial, Economic and Social Crisis. He was a member of the National Assembly during the IX Legislature, from 2002 to 2005, having held the position of coordinator of the Budget and Finances Committee, and during the X Legislature, from 2005 to 2009, having held the position of coordinator of the Budget and Finances Committee, and, between 2007 and 2009, he was the Chair of the Parliamentary Group of CDS-PP.

He is guest speaker and author of several books and articles on his areas of specialisation. He was partner in the Tax department of «JPAB – José Pedro Aguiar-Branco & Associados», from 2010 to 2014. He joined the Bar Association in 1995.