As of 2003 it is governed by the conservative Mayor Carsten Schwettmann.
After a short period under the governance of the Bishop af Bremen from 1421 to 1436 Delmenhorst returned under the custody of Oldenburg. Delmenhorst later was infamous for its robber-baronship under the count Gerd. Its reign ended in 1482 thanks to a siege laid to the castle under the leadership of the Münster bishop. Therefore the town now was under Münster authority until finally count Anton I won back the town as well as the castle in 1547.
When the last heir of Anton, Christian, died in 1647, Delmenhorst again fell under Oldenburg custody. As the Oldenburg regent of that time was a relative of the Danish king, Delmenhorst was under Danish control now.
In 1767 Delmenhorst was bought by Tsarina Katharina II, but was given up to Oldenburg in 1773. In 1777 Delmenhorst was declared a dukedom of Oldenburg. In 1806 a French and Dutch army occupied the territory; Delmenhorst was a part of the French empire under Napoleon from 1811 to 1813.
In the industrial age Delmenhorst experienced a great economical growth, thanks to Bremen. Since Bremen was in a different duty zone, merchants who wanted to Export manufactured goods outside of Bremen had to pay high customs duties. They therefore only exported the resources and produced their commodities in the surrounding villages. The industries arising were the Jute - a spinning works and weavery in 1871, the Delmenhorster Linoleumfabrik - a Linoleum factory, in 1882, the Norddeutsche Wollkämmerei und Kammgarnspinnerei or Nordwolle - another, bigger spinning works, and several others. The number of inhabitants quadrupled in these years.
In 1903 Delmenhorst was declared kreisfrei, meaning it was under its own regentship, not having to obey any other county. In the 1930s Great Depression the Nordwolle went bankrupt - nevertheless the town grew bigger, incorporating several smaller villages around it. On Kristallnacht in November 1938 the Synagogue was burnt down by the Nazis which came to power in Germany in 1933. After the Second World War Delmenhorst was in the British zone of occupation and had to deal with thousands of refugees from Eastern Germany, which now was occupied by the Soviet Union. In 1950, more than 57,000 people lived in Delmenhorst.
Since the 1960s there has been a steady decrease in employment, leaving more than 13% of the town's inhabitants unemployed and nearly 7% living on social welfare. In the year 2000 Delmenhorst was an outpost of the Hanover-based Expo 2000.
Another interesting place is the Burginsel (Castle Island), in which the old castle existed in medieval times. The construction was torn off during the 18th century. Today a park (called the Graft) occupies the grounds of the old castle.