From 1920 until the Second World War, Alan Bromly worked for Slough Council as Borough Engineer and Surveyor, having previously worked in Sutton Coldfield, Croydon and Godalming. This was a time of great change in Slough, with the population expanding from 16,000 to 66,000.

One of the major projects he undertook as architect was the Stoke Poges Lane Housing Scheme, in which over 200 houses were built at a cost considerably lower than houses being built elsewhere at the time. He was also responsible for the decision to buy up 42 acres of adjoining land, using it to build more houses, allotments and a cemetery. He designed and laid out the cemetery himself, which contained the unusual feature of 'rest gardens'.

Passionate about parks, Bromly argued that, considering the way that Slough's population was growing, more space should be set aside for parks and open spaces, and was involved in the decisions to purchase and develop Lascelles Playing Fields and Upton Court Playing Fields.

Alan Bromly was also responsible for several of Slough's landmarks. He was behind the decisions to purchase; a boggy meadow at Salt Hill, which was turned into the Salt Hill pleasure ground; a meadow off Montem Lane, which became the site of the open-air swimming pool; and a gravel pit and meadow at Salt Hill, which is where the new Town Hall and Central Depot were built.

He was also heavily involved in the improvements to the town's Waterworks in the 1920s.