Fort Widley is a polygonal Fort designed by William Crossman, an officer of the Royal Engineers, who was part of the staff of the Inspector General of Fortifications at the War Office The fort had enough accommodation for 250 men and there were 12 married quarters as well. The six-sided building was completed in 1870 at a cost of £93,980.

During the Victorian period Fort Widley and the other Portsdown Forts were used to accommodate various infantry units in the barracks

During the First World War it was used as a transit depot. During the Second World War it was modified to provide more accommodation. It was then used by a number of units before housing members of the Royal Corps of Signals and Auxiliary Territorial Service supporting the Navy command at Fort Southwick. In 1952 the fort became home to a bomb disposal squadron and a year later an emergency civil control centre for Portsmouth was constructed in the fort's magazine

In 1953, the Civil Defence Control was established for Portsmouth at Fort Widley, utilising the main magazine and the ground floor of the barracks block. This control was officially opened by the Director General of Civil Defence, General Sir Sidney Kirkman in January 1955. In 1961 it was taken over by Portsmouth City Council, and in 1973 it was used as the Hampshire County Council Standby Home Defence Control, the main one being in Winchester.

It continued to perform this function until the end of the Cold War and the Portsmouth District Council Emergency Centre, as it was by then known, finally closed in 1992

The fort became an activity centre for the Peter Ashley charity, providing a number of activities, including an equestrian centre. In September 2010 the fort was used to host a search and rescue exercise based around a simulated earthquake