Alexander Siemens was born in Hanover and was educated there and in Berlin. In 1867 he came to work in the Siemens Brothers' engineering works in Woolwich. There he was involved in laying underwater cables. He was a private in the Prussian Army during the war of 1870-1871 against France and then worked for William Siemens on various engineering projects (which included serving on the Cable Ship Faraday) both in England and abroad.
He became a British subject in 1878 and the following year took over the management of the electric light department of Siemens Brothers and oversaw the lighting of the Albert Hall and the Reading Room of the British Museum. He was President of the Institution of Electrical Engineers from 1894 to 1904 and of the Institution of Civil Engineers between 1910 and 1911. In 1915 he publicly stated his support for the Allies during the Great War.