He served as Minister of Agricultural, Food and Forestry Policies from 22 February 2014 to 13 March 2018, in the governments of Matteo Renzi and Paolo Gentiloni.

In 1994, he joined the Students' Movement, a left-wing organization, and in 1999 he was elected to the city council of Mornico al Serio for the Democrats of the Left (DS).

In 2015, Martina founded Left is Change, a left-wing social-democratic faction within the PD, allied with the centrist ones led by Matteo Renzi and Dario Franceschini.

Martina proposed a law against the so-called caporalato, that is the exploitation of unreported employment in agriculture, which had been a serious and widespread problem especially in Southern Italy.

[12] In the 2018 Italian general election, the centre-left coalition arrived third behind the centre-right alliance, in which Matteo Salvini's League was the main political force, and the Five Star Movement of Luigi Di Maio finished second.

[15] Renzi officially resigned on 12 March during PD's national directorate, and his deputy secretary Martina was appointed acting leader.