He began his political activity in the Social-Christian circles of the PUC, and was one of the co-founders of the Falange Nacional (FN, future Christian Democracy, DC) in 1938.

When senator and poet Pablo Neruda was banned by the Ley de Defensa Permanente de la Democracia (Law for the Permanent Defense of Democracy)—which banned the Communist Party of Chile and removed numerous voters from the rolls)—Tomić won the vacated seat in a by-election.

As leader of the progressive wing of the Christian Democrats, he was the party's presidential candidate in the 1970 election in which Salvador Allende won a plurality and, with Tomic's support, was ratified by parliament as president.

Of Croatian people descent, he was the fifth of seven children of the marriage between Esteban Tomic Dvornik (mayor of Calama in the 1930s, who inaugurated the present town hall of the commune) and María Romero García.

He also taught Economic Policy, Political Economy and Social Legislation at the Catholic University of Chile and the Polytechnic Institute.

In 1938, together with Eduardo Frei Montalva, Bernardo Leighton, Manuel Antonio Garretón and Rafael Gumucio, among others, he founded the Falange Nacional (FN).

Later he was senator for the provinces of Tarapacá and Antofagasta (1950-1953, in a complementary election, replacing Pablo Neruda who was disqualified by the[Law for the Defence of Democracy).

As a congressman he participated in various international activities: in 1951 he was a delegate to the Conference of American Foreign Ministers held in Washington; in 1955 he was received as a guest of honour by the Yugoslavian Government; in 1957, he was a member of the delegation that negotiated the Trade Treaty with Argentina and participated in the International Congress of São Paulo.

[4][1] In 1965 he resigned his seat as senator to accept the post of ambassador of Chile to United States, serving from 4 March 1965 to 13 April 1968.

He opened the political campaign with an emblematic phrase that is remembered for the turning point it marked within the party: I am not trembling to say it: either the democratic and popular revolution shaping an immense effort of people's participation for Chile to reach another horizon and a new destiny, or the institutional collapse will seriously divide Chileans against themselvesTomic ran a left-wing campaign that marked a break from the moderation of Eduardo Frei Montalva.

[6] British historian Alan Angell remarked that "Tomic's campaign looked almost as radical in tone and pronouncement as that of Allende", which attracted left-wing voters and progressive Catholics at the cost of right-leaning DC supporters, who often defected to Alessandri.

[6] Tomic performed exceptionally well amongst women and churchgoing Catholics, but failed to outcompete Allende in traditionally left-wing areas.

Serving as Ambassador of Chile to the United States, he was able to make connections with the American establishment and earn prestige amongst international leaders.

[13] According to George W. Grayson, likewise the most left-wing wing of Christian Democrats, the rebeldes (rebels), were willing to embrace Tomic despite his connections to the Johnson administration.

During his 1970 presidential campaign, Tomic stated that "it is impossible to be both a Catholic and a capitalist", called for a "noncapitalist path of development" and sought a union with socialist and communist parties, emphasizing that he envisioned "not a mere electoral alliance but a solid platform on which to base a revolutionary government.

Meanwhile, in his native Calama, the newest high school of the Municipal Corporation of Social Development (COMDES), created in 2002, bears his name.