Although it is not clear when exactly the building was constructed, the first references to it date to the middle of the 18th century. At the time John Mackintosh Square was known as The Parade due to its function as a parade ground. In a plan of the parade made in 1750 by Thomas James (published in 1770) the building is clearly shown, labelled “Main Guard Room,” with soldiers mustered outside it. It is also present on the 1753 plan as the “The Main Guard”.
The building was almost certainly severely damaged by the bombardment during the Great Siege – its proximity to the King’s Bastion (which took the brunt of bombardments) would have certainly meant this was the case. The second storey which can be seen on the building today was probably added when it was rebuilt.
According to the 1896 Gibraltar Directory it was still being used as a Guard House then. However by 1911 it seems to have fallen out of use. Between the two World Wars the building was used as the City Fire Station with alterations being made to the building to suit this purpose, such as the removal of the wooden floors on the ground floor so as to support the weight of the fire engines.
After World War Two the building was used for some time as a public bath-house until its use was changed again in the 2nd half of the 20th Century to Government department offices namely for the payment of public utility bills and road licensing.
The Main Guard has been home to the Gibraltar Heritage Trust since 2001. Throughout 2014 it has undergone a major renovation and has been restored to its former appearance, adding to the unique setting and heritage value of John Mackintosh Square. The ground floor of Main Guard is open to the public between 10am and 2pm every weekday. As well as the being the main reception area for the Gibraltar Heritage Trust, it boasts a temporary exhibition space, a shop that sells Gibraltar books and quality gifts and souvenirs and a quiet patio to the rear of the building.