She is the twin sister of Joana Mortágua, also MP of the Left Bloc, and distant cousin of Socialist politician Maria João Rodrigues.

Her appointment in September 2013 to the top positions on the list of candidates for deputies by the Political Commission of the BE was contested by a group of militants, who criticized the "technocratic criteria" that guided her choice.

In view of this, the BE confirmed that Mariana Mortágua was considered as the element that “would best serve the interests of the party in the Assembly of the Republic, due to her knowledge in the area of the Economy”, something that had “been felt since the departure of Francisco Louçã”.

[5] She later gained particular visibility in Portuguese politics after her performance in the parliamentary inquiry of Zeinal Bava and Ricardo Salgado, in the context of the bankruptcy of the BES bank.

[7] In September 2016, she stated that, "from a practical point of view, the first thing we have to do is lose the shame of looking for someone who is accumulating money" and that "we cannot be ashamed of having a social policy of this kind.