Aasen was born at Åsen in Ørsta (then Ørsten), in the district of Sunnmøre, on the west coast of Norway. His father, a small peasant-farmer named Ivar Jonsson, died in 1826. He was brought up to farmwork, but he assiduously cultivated all his leisure in reading, and when he was eighteen he opened an elementary school in his native parish. In 1833 he entered the household of H. C. Thoresen, the husband of the eminent writer Magdalene Thoresen, in Herøy (then Herø), and there he picked up the elements of Latin. Gradually, and by dint of infinite patience and concentration, the young peasant became master of many languages, and began the scientific study of their structure.
About 1841 he had freed himself from all the burden of manual labour, and could occupy his thoughts with the dialect of his native district, Sunnmøre; his first publication was a small collection of folk songs in the Sunnmøre dialect (1843). His remarkable abilities now attracted general attention, and he was helped to continue his studies undisturbed. His Grammar of the Norwegian Dialects (1848) was the result of much labour, and of journeys taken to every part of the country. Aasen's famous Dictionary of the Norwegian Dialects appeared in its original form in 1850, and from this publication dates all the wide cultivation of the popular language in Norwegian, since Aasen really did no less than construct, out of the different materials at his disposal, a popular language or definite folke-maal (people's language) for Norway. With certain modifications, the most important of which were introduced later by Aasen himself, but also through a latter policy aiming to merge this Norwegian language with Dano-Norwegian, this language has become Nynorsk ("New Norwegian"), the second of Norway's two official languages (the other being Bokmål, the Dano-Norwegian descendant of the Danish language used in Norway at Aasen's time). An unofficial variety of Norwegian more close to Aasen's language is still found in Høgnorsk ("High Norwegian").
Aasen composed poems and plays in the composite dialect to show how it should be used; one of these dramas, The Heir (1855), was frequently acted, and may be considered as the pioneer of all the abundant dialect-literature of the last half-century of the 1800s, from Vinje to Garborg.
Aasen continued to enlarge and improve his grammars and his dictionary. He lived very quietly in lodgings in Oslo (then Christiania), surrounded by his books and shrinking from publicity, but his name grew into wide political favour as his ideas about the language of the peasants became more and more the watch-word of the popular party.
Quite early in his career, in 1842, he had begun to receive a stipend to enable him to give his entire attention to his philological investigations; and the Storting (Norwegian parliament), conscious of the national importance of his work, treated him in this respect with more and more generosity as he advanced in years. He continued his investigations to the last, but it may be said that, after the 1873 edition of his Dictionary, he added but little to his stores. Aasen holds perhaps an isolated place in literary history as the one man who has invented, or at least selected and constructed, a language which has pleased so many thousands of his countrymen that they have accepted it for their schools, their sermons and their songs. He died in Christiania on September 23, 1896, and was buried with public honours.
Ivar Aasen-tunet, an institution devoted to the Nynorsk language, opened in June 2000. Their web page includes most of Aasens' texts, numerous other examples of Nynorsk literature (in Nettbiblioteket), and some articles, also in English, about language history in Norway.