Archaeological findings in the Frederikssund area tell us that people have lived here since the earliest stone age. In mediaeval times there was probably a small ferry crossing where the town is today. The area was uninhabited common land and the ferry was just a barge pulled by rope.

In the 1560’s when Frederik II launched the building of Frederiksborg Castle in Hillerød the ferry crossing became important as a route for building materials. A trading centre developed which, in 1573, was granted custom house status. In 1578 the citizens of Slangerup were granted permission to use it as a harbour, and built warehouses. A simple wooden jetty was built in 1587 but was replaced 35 years later by a stone wharf. In 1652 Frederikssund was named after the then king, Frederik III, although it was still under the administration of Slangerup.

Frederikssund became an urban society during the 1700’s when tradesmen settled in the rapidly developing town. At the same time Slangerup had a recession, losing its municipal status in 1809. This status was transferred to Frederikssund, making it one of the youngest towns in Denmark.

Frederikssund was well placed for shipping and in the mid-1800’s the harbour was modernised. At this time the first industrial companies were established and before long the village of Ude Sundby was incorporated in the town. In 1867 the ferry crossing was replaced by a pontoon bridge, Kronprins Frederik’s Bridge, which did service until it was replaced in 1936 by a permanent bridge bearing the same name. 

In 1879 the railway to Copenhagen was opened. Apart from benefiting freight transportation it also brought tourism to Frederikssund. The new connection meant that in the 1900’s, Frederikssund and the Fjord Country became an attractive goal for the Sunday outings and holidays of Copenhageners.

In 1740 there were 300 inhabitants in Frederikssund. In 1930 there were 3,200, and the population continued to grow right through the 1900’s thanks to the town’s continued development and the many people who moved here.

Prior to the municipal reform of January 1st 2007 there were almost 20,000 inhabitants in Frederikssund municipality. Now it is part of a large new municipality, having merged with Slangerup, Skibby and Jægerspris, and has in 2008 around 45,000 inhabitants. This makes Frederikssund the third-largest municipality in North Zealand. It is expected that the population will continue to grow and that commerce will develop so that Frederikssund will, for years to come, be a significant player in the development of the Copenhagen Region.