The original family name (Di Tommaso) was changed by De Tomaso upon being nominated a stenographer in the National Congress of Argentina when he was 24.

He later married Isabella Ceballos Arellano, from a prominent family of land-owners, with whom he fathered four children: Marcelo, Jaime, Alejandro, and Carlos.

[2] De Tomaso was a member of the Socialist Party since 1907 and was elected to the National Chamber of Deputies in 1914 for the City of Buenos Aires.

At Congress he interested on issues related to foreign policy and defense for the Socialist Party, but also participated in important debates on issues of economic and social legislation and contributed to the 1921 reform of the Criminal Code.

At the head of the new party, he sided in favor of the 1930 coup led by Agustín Pedro Justo, who became President in February 1932.