In 1860 a civil engineering contractor was awarded the contract to build Nothe Fort. The first stage was the construction of a sea wall that would allow a level site to be formed on the sloping ground at the end of the Nothe Peninsula. However, due to financial problems, they failed to complete the wall. This responsibility and the construction of the fort were then given to 26 Company Royal Engineers.

The fort is constructed on three levels. The lowest level, Magazine Level, was originally designed to store gunpowder and shells. The middle level, Gun Deck Level, was designed to accommodate 12 heavy muzzle loaded cannons and provide accommodation for the soldiers manning the cannons. The top level formed the ramparts and provided a raised platform that could be used to fire muskets and light muzzle loaded cannons during an attack on the fort.

The fort was commissioned in 1872 at a cost of £120,000

The first soldiers to be garrisoned at Nothe Fort were No.2 Battery Royal Artillery (Tatton-Browns) a specialist gun emplacement unit trained in the handling of cannons up to 18 tons. They were responsible for installing the original cannons; these were 2 x 64 pounders, 4 x 9 inch cannons and 6 x 10 inch cannons.

The cannons were all rifled muzzle loaded (RMLs), with spiral grooves cut into the barrel. The groves engaged with lines of lugs positioned along the length of the shells. When a cannon was fired the shell would begin to spin as it travelled along the length of the barrel, this spinning motion improved the range and accuracy of the shell.

Seven of the original cannons were replaced in the 1890s by massive 12.5 inch cannons, each weighing 38 tons and capable of firing a 800 pound shell over a range of three and half miles. The shells for these cannons were fitted with a soft metal ring instead of lugs. When the cannons were fired the soft metal was forced in to the rifling and this caused the shells to spin.
 
As Portland Harbour grew in importance and became the main base for the Channel and later the Atlantic fleets, so Nothe Fort remained an important element in the defences of the naval base.

With the advent of breach loading (BL) guns the 12 riffle muzzle loaded (RML) cannons were removed and 3x 6 inch guns were emplaced on the Ramparts. These new guns had a greater accuracy and better rate of fire than the 12 muzzle loaded guns. The armour piecing shell of the 6 inch gun weighed 100 pounds and had a range of 10 miles.

In order to accommodate the 6 inch guns it was necessary to undertake extensive structural work to support the weight of these guns on the Ramparts. In addition to the structural work a considerable amount of earthworks was undertaken to protect the guns from enemy fire. In order to convey the shells and cartridges from the magazines to the gun emplacements a series of hoist were installed
 
Nothe Fort did not see action against an enemy until World War 2, when the main threat came from the air. Until then the heavy 6 inch guns had mainly been fired in training and in competitive events. However in July 1940 two ships failed to identify themselves and were fired on.

They quickly turned on their lights to reveal themselves as refugees from the Channel Islands. In 1938 alterations were made to the fort to allow it to be used as a central anti-aircraft ammunition depot for the south west, this included installing an electrically operated hoist and constructing a loading platform.

During World War 2 the fort was equipped with a Vickers pom-pom which was sited on a platform built on the north-west corner of the fort; this was later replaced by a 40 mm Bofors. The main anti aircraft guns on the Nothe Peninsula were emplaced in Nothe Gardens, in the area that is now the car park nearest the fort. The anti aircraft emplacement in Nothe Gardens consisted of 4 x 3.7 inch Vickers.