After studying law at the National Institute, where he also served as rector (1835–40), Montt was elected to the Chilean Congress in 1840. He served as minister of the interior and minister of justice under President Manuel Bulnes (Bulnes, Manuel) (1841–51).

In 1851 Montt won the presidency, but the liberals thought his election was fraudulent and instigated an armed revolt, which was quickly subdued. Montt represented the conservative oligarchy and was authoritarian and inflexible in his beliefs, but he also worked for the economic and social progress of his nation. He angered the conservatives when he asserted the state's right of patronage in Chile's Roman Catholic Church and when he supported the abolition of restrictions on the sale or bequeathing of landed estates. His administration made advances in commerce and banking, codified Chilean laws, strongly promoted public education and immigration, and colonized the area south of the Bío-Bío River.

Near the end of his second term, when Montt indicated a preference for Antonio Varas, his minister of the interior, to be his successor, liberals again staged an armed uprising. Montt again subdued the revolt but pacified the liberals by shifting his support to José Joaquín Pérez, who was a moderate. On giving up the presidency in 1861, Montt became president of the Supreme Court, a post he held at the time of his death.