As a young man, Altamirano won medals in the high jump event at the 1946 and 1947 South American Championships in Athletics.

On July 26, 1971, having been elected General Secretary of the Socialist Party, he called on President Salvador Allende to dissolve the National Congress.

In 1973, sectors of the Chilean Navy tried to convince leaders of the far-right paramilitary organization Patria y Libertad to assassinate him.

After the 1973 coup, Altamirano went into exile: after first fleeing to Cuba, he spent the years of Augusto Pinochet's dictatorship in East Berlin (1974–1979) and in Paris (1979–1992).

During his years in exile, Altamirano would renounce many of his earlier radical political standpoints, instead becoming more of a moderate social democrat.