Always dedicating himself professionally to the insurance companies sector, in 1932 he was president of the Federation of Young Christians and director of the newspaper El Matí,[1] close to UDC.
At the outbreak of the civil war in 1936, he left for Trieste when he felt his life threatened in Catalonia, and later returned to Spain to volunteer in Burgos to fight on the side of the rebels.
[3][4] In 1943 he founded the Benèfica Minerva, which was clandestinely dedicated to collective patronage, and in which the lawyer Pere Puig i Quintana, at the time Millet's right-hand man, collaborated.
[5] In 1947, Millet was secretary of the Comissió Abat Oliba and president of the board of directors of the Banco Popular Español -until 1957- and of the Compañía Hispano Americana de Seguros y Reaseguros.
As shareholder and president of Banco Popular, he made disappear "the merchant and shopkeeper mentality, linked, in this case, to the Madrid artisans" in the attempt to create "a modern bank, in every sense of the word".