In 1922, he started working at the
Central Archives of the Estonian State, where he organized the older archives of
Saaremaa: the archives of the knighthood, the , the audit committees, the consistory, the office of the supreme church leader, and so on. In 1930 he began working as the acting director of the Narva City Archives and
Narva Museum, replacing the previous city archivist (1863–1933), and in 1931 Soom was confirmed as the director of both institutions. Soom mainly studied the economic history of the city of Narva, especially during the
Swedish period. Together with others interested in Narva's history, Soom founded the Narva Historical Society in 1931, the most important achievement of which was the publication of the first volume of
Narva ajaloo (Narva History) written by
Arnold Süvalep in 1936 and edited by Soom. In 1939,
Carl Sarap's photo album
Vana Narva (Old Narva) was also published, the historical overview of which was written by Soom. At the recommendation of the Narva Historical Society, the
bastions and other defensive structures of Narva were repaired, and the Swedish-era
Victoria, , , and bastions were cleaned and repaired and opened to tourists. In 1937, the was taken under
heritage protection. On September 1, 1940, after the occupation and annexation of Estonia, Soom was appointed deputy director of the Estonian SSR State Central Archives, as an assistant to the director of the Central Archives, . On February 17, 1941, after Ney had moved to Germany, Soom was appointed acting director of the Estonian SSR State Central Archives. During the
German occupation, Soom was the director of the Tallinn Central Archives of the
Estonian Self-Administration and the
State Library connected to it from September 1941 to September 1944. In 1944, he fled to Sweden, where he worked at the
National Archives of Sweden until his retirement in 1966. In addition to archival work, Soom conducted research in economic and social history. Soom's work was later continued by
Enn Küng. Arnold Soom's greatest work at the National Archives of Sweden was the reorganization and new cataloging of the Livonica II collection, which contains the 16th- and 17th-century archives of the Swedish Baltic provinces of
Estonia,
Ingria,
Livonia, and
Saaremaa. Soom wrote five volumes, over a hundred research articles and reviews, and extensive journalism. Soom was one of the founding members of the in 1945 and was a member of the board of the society until 1975. He was also active in the , the , the Swedish Baltic Institute, and the
Baltic Historical Committee (in
Göttingen, Germany). In 1966 he was elected a member of the Turku Historical Society ('''') and the
Royal Society for Publication of Manuscripts on Scandinavian History. ==Publications==