He left Barga again on 19 June 1859 in order to join up with Garibaldi who was leading a rapidly expanding volunteer army at Valtellina, the area surrounding Sondrio, close to the 3-way alpine Lombard border with Austria and Switzerland. Mordini now participated in the War of Independence in the élite "Cacciatori delle Alpi" ("Alpine Hunters'") unit, under the command of General Giacomo Medici. He remained in the mountains only a few weeks, however. By 11 July 1859 he was in Turin, pleading the case (and voting) for an immediate annexation of Tuscany to Sardinian-Piedmont at the negotiations being held to finalise terms for the , which was finalised the next day. He was still arguing in favour of such a development as a member of the "Tuscan Assembly", to which he was elected in August 1859, urging that diplomatic procrastination be set aside and that the Assembly itself might decide the matter. In the end the annexation of Tuscany to the Kingdom of Sardinia-Piedmont was deferred till the next year, taking place at the same time as the annexation of other parts of central Italy, and only with the backing of a referendum, which was held on 12 March 1860. After that, in the election of , he was elected to parliament by the voters of the Borgo a Mozzano.