Educated at the University of Padua, in 1428 he sought a position with the papal diplomatic corps, but was turned down as too young. In 1429, he accepted a position teaching rhetoric at Padua, but was compelled to resign after publishing an open letter mocking the scholastic method of jurisprudence.
After leaving Padua, in 1433 Valla made his way to Naples, and the court of Alfonso V of Aragon, where he became Alfonso's Latin secretary. He was summoned before the Inquisition on account of his public statements about theology, including one in which he denied that the Apostles Creed was composed in succession by each of the twelve Apostles, but these charges were dropped.
Valla is remembered chiefly for two works. The first is the De elegantia linguæ Latinæ, in which Valla put the movement of the Humanists to reform Latin prose style to a more classical and Ciceronian direction on a scientific basis. Valla's work was controversial when it appeared, but its arguments carried the day. As a result, humanistic Latin sought to purge itself of post-Classical words and features, and became stylistically very different from the Christian Latin of the European Middle Ages. This was thought to be a major improvement in style and elegance in Latin usage. However, its ultimate result was that the approved style of humanistic Latin, purged of neologisms and newly developed meanings for words, was much harder to write correctly than the workaday Latin based on the Vulgate which was used as a learned but still living language by lawyers, physicians, and diplomats. Valla hastened the process of converting literature to the vernacular languages by making Latin much more difficult to use and learn.
Valla's knowledge of classical Latin style, however, was put to good use in an essay he published in 1440, De falso credita et ementita Constantini donatione declamatio, in which he demonstrated that the forged "Donation of Constantine" could not have possibly been written during the time of the Roman Empire. Alfonso of Aragon was currently involved in territorial conflict with the Papal States, and as his secretary Valla was motivated to show that the Donation, often cited in support of the temporal power of the Papacy, was a forgery. His argument, however, was convincing despite these interests, and as a result the falsity of the Donation is generally conceded.