He was a leading voice of the Científicos, "the scientists" who were the intellectual leaders during the regime of Porfirio Díaz.

Together with his fellow young students, Sierra responded with patriotic fervor to the invasion of his country, and became a lifelong militant liberal.

His most enduring works are sociopolitical histories (at times verging on memoirs)[citation needed] of the era of Benito Juárez and Porfirio Díaz, particularly his political biography of Juárez and his Evolución política del pueblo mexicano.

Elected to several terms as a representative in the federal Chamber of Deputies, Sierra also served the government in various posts.

President Álvaro Obregón's Minister of Public Education, José Vasconcelos republished Sierra's Historia Patria for use in schools.