Mon was born in Oviedo and was the eldest son of Miguel Mon y Miranda and Francisca Menéndez y de la Torre. His only sister, Manuela, was married to the AsturianPedro José Pidal, 1st Marquis of Pidal, another prominent politician who served several times as prime minister.
When the moderates came back to power in 1844, a period started known as the Moderate Decade, Mon was called by the new prime minister, Ramón María Narváez, 1st Duke of Valencia, again as minister of finance. He held this post from 1844 to 1845 and carried out the tax reform of 1845, which established the basis of the current tax system of Spain. It was executed in collaboration with Ramón de Santillán, and it is popularly known as the Mon-Santillán reform.
After the downfall of the moderates, the unionistLeopoldo O'Donnell, 1st Duke of Tetuan, offered him several ministerial portfolios, which he always declined. Mon preferred to be in posts away from the first line of political life, such as ambassador to the Holy See or to France. However, he returned to active politics in 1864, when he replaced Lorenzo Arrazola y García as prime minister, but his cabinet lasted only nine months because of the social and political instability. Five years later, the Spanish Glorious Revolution took place.
Comin, Francisco, Alejandro Mon y Menéndez (1801−1882). Pensamiento y reforma de la Hacienda, Instituto de Estudios Fiscales, Madrid, 2001. ISBN978-84-8008-095-8
Fernández de la Mora, Gonzalo (2001), Mon en su siglo in galeon.com (in Spanish)