He was born in Erlangen (Bavaria), and was trained in the Academy of Fine Arts Nuremberg and Munich. Haag practised first as an illustrator and as a painter, in oil, of portraits and architectural subjects; but after he settled in England, in 1847, he devoted himself to water colors, and was elected associate of the Royal Society of Painters in Water Colours in 1850 and member in 1853. He traveled much, especially in the East, and made a considerable reputation by his firmly drawn and carefully elaborated paintings of Eastern subjects. Some of his depictions of the Middle East are at the Israel Museum's collection. Towards the end of his professional career Carl Haag quit England and returned to Germany where he died in Oberwesel.
See A History of the Old Water-Colour Society, now the Royal Society of Painters in Water Colours, by John Lewis Roget (2 vols, London, 1891).