Born Carl Wilhelm in Lenthe, Germany, Siemens was the son of a tenant farmer and a younger brother of Ernst Werner Siemens. He was educated at Göttingen and first visited Britain in 1843 as an agent of his brother Ernst Werner. He settled in England shortly afterwards, becoming a naturalized citizen in 1859.

Siemens worked in two main areas, namely, heat and electricity. In the age of Joule he was aware of the potential value to be gained by conserving heat. Early attempts to redesign the steam engine proved impractical. More successful was his introduction, aided by his younger brother Friedrich (1826–1904), of the regenerator furnace (1856). In the Siemens furnace the hot combustion gases were not simply discharged into the air but used to heat the air supply to the chamber. The process was first used in the manufacture of steel by an open-hearth process known as the Siemens–Martin process (after the French engineer Pierre Blaise Emile Martin, 1824–1915) in the 1860s. It proved to be the first serious challenge to the Bessemer process and by the century's end had become the favored method of steel production. On the strength of this a steel foundry was opened at Landore, South Wales, in 1869. As it failed to prosper it was abandoned in 1888.

He was more successful with his work in electric telegraphy. Siemens designed the cable-laying ship Faraday for laying a new trans-Atlantic cable in 1874. He also worked on electric lighting and on the Portrush electric railway in Northern Ireland. He died suddenly from a heart attack in 1883. When, forty years before, he had first arrived in Britain his English had been so poor that he looked for legal advice from an undertaker. Since then, it was said of him, he made three fortunes: one he lost, one he gave away, and one he bequeathed to his brothers. The electrical unit of conductance, the siemens, is named in his honor.