His older brother, Juan Menéndez Pidal, whom he outlived by more than fifty years, was also a literary scholar of the folk poetry of Asturias.

During the 1920s Menéndez Pidal published in rapid succession a series of major studies: Poesía juglaresca y juglares (1924) traced the development of minstrel poetry in medieval Spain.

Orígenes del español (1926), a landmark in Romance linguistics, retraces the pre-literary phase of the Ibero-Romance dialects, and the "triumph" of Castilian.

However, from this period is his sweeping essay "Los españoles en la Historia," a study that traces the struggle between liberals and conservatives in the entire course of Spanish history.

He also summarised his findings on the ballads in Romancero Hispánico: Teoría e historia (1953) and applied his theory of the origins of epic poetry to French literature in La Chanson de Roland y el neotradicionalismo (1959).

Menéndez Pidal worked for many years on a comprehensive history of the Spanish language, which he could not complete in his lifetime; the two volumes have been published posthumously as "Historia de la lengua española" (2005).