His parents were Josep Tomàs i Boix and Àngela Piera i Trias, He obtained a degree in Law at the University of Barcelona in 1920.

[2] Tomàs was a member of Republican Action of Catalonia (Acció Republicana de Catalunya, ARC), a party created by left-wing members who split from Catalan Action (Acció Catalana, AC), during the 1923–30 dictatorship of Miguel Primo de Rivera.

In 1930 he represented the ARC in the Revolutionary Committee of Catalonia (Comitè Revolucionari de Catalunya), which brought together various political groups opposed to the dictatorship.

In the early part of 1931 he participated in the process of recombining the ARC and AC, leading to the formation of the Catalan Republican Party (Partit Catalanista Republicà, PCR).

[2] In 1933 Tomàs and other Catalan leaders such as Antoni Rovira i Virgili, Carles Pi i Sunyer and Josep Sunyol joined the Republican Left of Catalonia (Esquerra Republicana de Catalunya, ERC).

[2] Tomàs was elected a member of the Cortes Generales (parliament of Spain) on 19 November 1933 for the province of Barcelona on the Republican Left of Catalonia platform.

[4] In the sessions of the Cortes during the months before the outbreak of the Civil War he was one of the presidents of the Left parliamentary group.

[2] Tomàs was appointed Minister of Labor, Health and Social Welfare in the first government of Francisco Largo Caballero (September–November 1936).

He was part of the Junta de Auxilio a los Republicanos Españoles (JARE), which assisted exiles, and was its secretary in Mexico.

[9] In December 1941 Tomàs and other members of the Esquerra Catalana in Mexico (Jaume Aiguader, Joan Casanelles, Pere Ferrer, Joan Lluhí and Josep Mascort) requested that the president of the Cortes meet the Standing Committee of the House and resume the activity of the Spanish Republic in Mexico, since that was impossible in Europe, which was occupied by the military forces of the totalitarians.

[10] He participated in the Spanish parliament in exile, and in 1945 promoted and presided over a federalist parliamentary group of Catalan, Basque and Galician deputies.