In 1920 Palazzo Marino returned to state ownership, in exchange for the Royal Palace. In 1926 it was finally handed over to the City Council. During WWII, Gian Giacomo Gallarati Scotti was Mayor of the city, before being replaced by commissioner Giorgio Boltraffio after the 25 July.

In August 1943, bombing caused extensive damage to the palace, particularly to the Hall of Honour and the building's restoration was only completed on 12 April 1954 under the direction of Arrigo Buonomo. During this time, with the advent of the RSI (Italian Social Republic), Piero Parini was appointed Mayor. He was succeeded by Guido Andreoni (May to September 1944), Giuseppe Spinelli (September 1944 to January 1945) and Mario Colombo (January to April 1945). After the Liberation, the CLN (National Liberation Committee) nominated Antonio Greppi as Mayor, and he was then confirmed in the April 1946 elections (in September he left the Constituent Assembly, to which he had been elected).

Greppi, who had already fought in WWI, joined the PSI (Italian Socialist Party) in 1919. Under the regime, he practised law as a 'poor man's lawyer'. In 1938, he was incarcerated and continued his clandestine activities with the socialists. In 1943, he participated in the founding of the Milan CLN, then went to Switzerland. On his return to Italy he collaborated with the Resistance in the VIII Matteotti brigade. He also lost his son Mario, who was killed by fascist militia on 23 August 1944.

As Mayor of the city's reconstruction, Greppi was mainly concerned with ensuring decent living conditions for the population and restoring infrastructure and basic services. He worked on the accommodation of displaced persons, building more than 100,000 rooms, as well as the reconstruction of La Scala Theatre and creation of the Piccolo Theatre. Politically, his mediating role in resolving difficult situations, such as the 'Parini loan' and the occupation of the prefecture in 1946, was crucial. In 1949 he joined the PSDI (Italian Democratic Socialist Party), but left in 1953 in the face of the 'fraud law'. He later returned to the the party, where he was a deputy from 1958 to 1968. A writer and playwright, he also wrote a diary about his experience as Mayor.