In 1835 Jón was appointed a Fellow at Den Arnamagneanske samling (the Árni Magnússon manuscript collection) in Copenhagen. Den Arnamagnæanske Kommission (the Árni Magnússon Commission) deemed him the best-qualified applicant, as he had acquired extensive knowledge of documents and manuscripts when he was secretary to the Bishop of Iceland. The Commission also judged that Jón was very accurate and conscientious, and wrote a fine and clear hand – which was an important factor, as the work involved copying manuscripts and documents in the collection. Jón performed his work in an exemplary fashion, and was paid bonuses for his diligence, keenness and hard work. In 1848 Jón was elected secretary of the Árni Magnússon Commission, a position he retained for the rest of his life. Jón compiled a detailed catalogue of the manuscripts in the collection, produced editions of Icelandic annals and the Prose Edda of Snorri Sturluson, made copies and comparative studies of historical manuscripts, prepared an annual report on the Commission’s work, supervised the work of Fellows, etc.
In the summer of 1841 Jón Sigurðsson went to Uppsala and Stockholm in Sweden with theology student Ólafur Pálsson, on behalf of the Árni Magnússon manuscript collection and Det kongelige Nordiske Oldskriftselskab (the Royal Society of Northern Antiquaries). They were to study and catalogue Icelandic manuscripts in collections in the two cities. They spent three very fruitful months in Sweden: they found a large number of previously-unknown Icelandic manuscripts, made copies of important ones, and Jón catalogued them in detail.
In 1851 Jón was elected chair of Hið íslenzka bókmenntafélag (the Icelandic Literary Society). Founded in 1816, the society has published books and periodicals about Iceland and its history and culture. During his time as chair of the society, Jón was an energetic leader: the society acquired an extensive collection of Icelandic manuscripts, and published a large number of books on diverse subjects. The vigorous activities of the society at that time led to an explosion in membership, from 160 in 1841 to 794 in 1877.
In Jón Sigurðsson’s day it was not unusual for Icelanders to own old manuscripts of various kinds: copies of old poems and verses, whole manuscripts of sagas, genealogical records and diverse interesting information. Jón felt it was vital to preserve these manuscripts, which were at risk of being lost for ever, and thus he started to accumulate his own personal collection. This eventually comprised over 1,300 manuscripts, which are now preserved in the National Library and University of Iceland.
On Jón’s initiative, publication began of legal documents relating to Iceland, under the titleLovsamling for Island (Legal Code for Iceland). Twenty-one volumes were published between 1851 and 1889. Never before had legislation, directives and other legal instruments of different periods been collected together to throw light on Icelandic judicial and political history in the period 1096 to 1874. The project thus met an urgent demand and need for access to such information.