Fet was established as a municipality on 1 January 1838 (seeformannskapsdistrikt). The new municipality ofRælingen was separated from Fet on 1 July 1929. The part ofEnebakk municipality lying east of lakeØyeren was transferred to Fet in 1962. On 1 January 2020 Fet was merged withSkedsmo andSørum municipalities to formLillestrøm municipality.
Thecoat-of-arms is from modern times. They were granted on 19 December 1986. The arms show alogging hook, which was used in the area to haul the logs cut in the surroundingforests. Thelogs were transported over the streams and lakes to thesawmills. The people used long poles with hooks to haul the logs. The colours of the arms are green and silver because the name is derived from an old word describing a field (green) near a lake (silver).[4]
Fet is located on the eastern side ofLake Øyeren and includes the point at which theRiver Glomma enters the lake. The inlandriver delta that has formed is the largest inEurope and reaches the opposite shore of the lake, across its short axis. Until 1985,logs floated down the river were brought onshore here before the river entered its delta, and were then transported onwards by rail. Today, many of Fet's floating buildings have been preserved and converted into amuseum and cultural centre.