William G. Brounger was appointed as an apprentice railway engineer under Sir Charles Fox (1810-1874) in 1837. He became an associate of the (British) Institution of Civil Engineers in 1847 and a member in 1864. In 1851 he worked with Fox on the design of the large London Exhibition hall, later known as the Crystal Pallace. Thereafter he was in charge of the laying out and building of the Sjaelland railway in Denmark. In 1857 he was appointed by the Cape Town Railway and Dock Company to build the first railway line in the Cape Colony (and the second to be opened in southern Africa), from Cape Town to Wellington. Arriving in October that year he finished surveying the line in April 1858 and returned to England. Towards the end of the year he came back with six other engineers to assist him and help him train local workers. The six were J. Bisset*, W. Perrolt, A. Priestley, J.W. Roberts, W.B. Taylor and E.W. Young*.
Brounger was the resident engineer during construction of the railway. The first section, to Eerste Rivier, was opened in February 1862 and Brounger also supervised the operation of this section. The line to Wellington was opened in November 1863, and a line from Salt River to Wynberg the next year. Brounger supervised the operation of the line to Wellington until June 1864, when he returned to England.
In 1870 he again arrived at the Cape, joining the government service the next year. He spent some years planning and surveying railway lines in the Eastern Cape with his son, Richard Ernest Brounger. In 1873, when the government of the Cape Colony took over the railways, Brounger was appointed Colonial Railway Engineer and was in charge of all railway construction and management. He organised the replacement of the colony's fast wearing wrought iron lines with newly developed steel rails, experimented with various kinds of wooden sleepers, and supervised the construction of lines linking Cape Town, Port Elizabeth, and East London to Kimberley. During his term of office the lines from Cape Town and Port Elizabeth were almost completed to De Aar, and for a few months the junction there was known as Brounger's junction. The line from East London was completed to Sterkstroom in his time, while branch lines were completed to Malmesbury and Grahamstown. His health deteriorated in 1883 and he retired to Britain the next year and settled in Guildford for the rest of his life.
In 1885 Brounger read a paper on "The Cape Government Railways" before the Institution of Civil Engineers in England. It was published in the Minutes of the Proceedings and earned him the Institution's Telford medal. He was an outstanding railway engineer of his time and was known for his thoroughness. His son also served as an engineer with the Cape Government Railways from 1870 to 1896. The elder Brounger served on the committee of management of the Cape Town botanical gardens from 1873 to 1881, and represented the Cape Colony at an international phylloxera congress in France in 1881. In 1847 he married Sarah Bethia Roberts.