Works on the line began in 1957, and the first part was opened on 1 November 1964,[4][5] running from Sesto Marelli to Lotto station.

The line runs underground from the northern suburb of Sesto San Giovanni to the city centre, then to the western district with two different branches, one northwest to Rho, the other to the west to Bisceglie.

On 6 April 1952 the city administration asked for a project of a metro system and on 6 October 1955 a new company, Metropolitana Milanese, was created to manage the construction of the new infrastructure.

The line from Lotto to Sesto Marelli (21 stations) opened on 1 November 1964, after seven years of construction works.

[2] In 2015, the Leonardo trains were introduced, replacing the original rolling stock, which was subsequently retired and dismantled.

On November 17, 2024, the first of 21 trains produced by Hitachi Rail at the Reggio Calabria plant was delivered for Line 1.

[15] The new section will be 1.9 km (1.2 mi) long with 2 stations (Sesto Restellone and Monza Bettola), entirely underground.