Professor of medicine at the Univ. of Havana, Grau San Martín opposed Gerardo Machado. He then joined with student radicals and the military junta that ousted Carlos Manuel de Céspedes and was named provisional president. Grau was in turn removed from office by a coup led by Fulgencio Batista y Zaldívar.

He lost the presidential election to Batista in 1940, but he won in the free election of 1944 and as president launched a widely hailed program of social and economic readjustment. Corruption and nepotism were soon rife, and former supporters turned against him. A booming economy, however, favored the election (1948) of Grau's candidate, Carlos Prío Socarrás.

Grau's political influence remained strong during Prío's regime, and when Batista ousted Prío (1952) in a second coup, he retained a measure of popularity. In 1954 he ran against Batista but charged fraud before the election and retired. After Batista's ouster (1959) by Fidel Castro, Grau remained in Cuba. He was a known homosexual.