He entered politics in 1903 at the urging of Manuel Candamo (the then-leader of the Civilista Party) and also of José Pardo, who was prime minister.

Pardo offered the position of Prime Minister to Leguía, who accepted and remained so until 1907, when he resigned to run for the presidency the following year.

In 1908 he succeeded José Pardo (a succession event that would occur again in 1919) after being elected president for the first time by an alliance of the Civil and Constitutional parties.

Some of Leguía's first actions were to institute social and economic reforms in an attempt to industrialize Peru and turn it into a modern capitalist society.

On 29 May 1909, a group of citizens (supporters of Piérola's Democratic Party) managed to force their entry into the Palacio de Gobierno demanding the resignation of Leguía.

Leguía changed the Peruvian constitution (which had the longest continuance since 1860), and promulgated a new one in 1920, which was more liberal than its predecessor and provided more civil guarantees and unlimited reelections.

Another important political figure that would emerge during this era was José Carlos Mariátegui, leader of the Communist Party of Peru.

Among the positive initiatives that occurred during Leguía's second term was a program to modernize Lima by planning and starting public works through various loans.

The Great Depression had drained foreign investment in Peru and after eleven years in power, Leguía's government was overthrown.