Lastra was born in Ribadesella, a municipality of the Spanish region of Asturias. She started the studying cultural anthropology but she did not finish the degree.[2]
Lastra joined the Socialist Workers' Party at the age of 18 and she was appointed Secretary-General of the Socialist Youth of Asturias the following year, in 1999, being in the position until 2004 when she was appointed Secretary for Social Movements and NGOs. Between 2008 and 2012 she served as Secretary for Local Policy of the Asturian Socialist Federation.
After the 2016 general election, the possibly of a third consecutive election was imminent and the Secretary-General Sánchez proposed to the Federal Committee of the PSOE (the executive body of the Party) the possibility of holding an Extraordinary Congress with an urgent nature to decide if they should allow the Second Rajoy Government or rejecting it (the position of Sánchez) by asking the voters. The position of most of the members of the Federal Committee was to refrain and allow Mariano Rajoy to form a new government, rejecting Sánchez's request and forcing his resignation. After his resignation, a Caretaker Commission take control of the Party and Lastra publicly rejected what the Federal Committee did.[4]
She was one of the decisive supports in the decision of Pedro Sánchez to attend the 2017 leadership election to regain the leadership of the PSOE and played an essential role in the campaign of the candidate, based on the rejection of the ideological project of the People's Party and the importance of the socialist bases to recover the party. After Sánchez was re-elected as Secretary-General with more than 50% of the votes, the 39th PSOE Congress was held in which Lastra was named Deputy Secretary-General becoming second-in-command to Sánchez and Deputy Spokesperson of the Socialist Group in the Congress of Deputies.[5]