[3] Rask was born to Niels Hansen Rasch and Birthe Rasmusdatter in the village of Brændekilde near Odense on the Danish island of Funen.

One of his friends from Latin school, Niels Matthias Petersen (1791–1862), who went on to be the first professor of Nordic languages at the University of Copenhagen, later remarked that "His short stature, his lively eyes, the ease with which he moved and jumped over tables and benches, his unusual knowledge, and even his quaint peasant dress, attracted the attention of his fellow students".

His teacher, Jochum E. Suhr, loaned him a copy of Snorri Sturluson's Heimskringla in Icelandic, and the rector, Ludvig Heiberg, gave him a new translation of the same work as a prize for his diligence.

Although he was not particularly religious and even had expressed serious doubts, he signed up as a student of theology, although in practice he simply studied the grammar of various languages of his own choosing.

[1] In 1811, the Royal Danish Academy of Sciences and Letters put out a call for a prize essay on the topic of language history that would "use historical critique and fitting examples to illuminate the source whence the old Scandinavian tongue can be most probably derived, to explain the character of the language and the relations that it has had through the middle ages to the Nordic as well as Germanic dialects, and to accurately ascertain the basic tenets upon which all derivation and comparison of these tongues should be constructed.

When he returned to Denmark, he was recommended to the Arnamagnæan Institute, which hired him to edit Björn Halldórsson's Icelandic Lexicon (1814), which had long remained in manuscript.

Because of this, Rask envisioned a trip to India to study Asian languages such as Sanskrit, which was already being taught by philologists such as Franz Bopp and Friedrich Schlegel in Germany.

[2] In October 1816, Rask left Denmark on a literary expedition funded by the monarchy to investigate Asian languages and collect manuscripts for the University of Copenhagen library.

In 1819, he left Stockholm for St. Petersburg, Russia, where he wrote the paper "The Languages and Literature of Norway, Iceland, Sweden and Finland," which published in German in the sixth volume of the Vienna Jahrbücher.

In 1820, he traveled from Bushehr, Persia to Mumbai, India (then called Bombay), and during his residence there, he wrote (in English) "A Dissertation on the Authenticity of the Zend Language" (1821).

Rask's Anglo-Saxon, Danish and Icelandic grammars were published in English editions by Benjamin Thorpe, Þorleifur Repp and George Webbe Dasent, respectively.