Distrusting the teaching offered in Spain's universities at the time, Leandro grew up in the rich literary environment of his father and became an admirer of Enlightenment thought.

In 1790 he published his first comedy El viejo y la niña (The Old Man and the Young Girl), a sombre work which attacked the consequences of arranged marriages between people of differing ages.

A supporter of Joseph Bonaparte, whose rule had allowed far more expression of liberal thinking than Spain's Bourbon monarch Carlos IV was willing to tolerate, Moratín was given the post of royal librarian.

However, his 1805 comedy El sí de las niñas (The Maidens' Consent) was denounced upon the reinstatement of the Inquisition when Ferdinand VII regained the throne after the fall of the Bonapartes, and he had to abandon playwriting and was forced into exile in France.

However, at the turn of the 20th century, his remains were brought back to Spain for interment in Madrid's Panteón de Hombres Ilustres (Pantheon of Illustrious Men).