Knut A. Carlson entered the civil service of the Cape Colony in February 1888 and was posted to Harkerville, near Knysna, as a forester. In 1892 he was transferred to Kokstad as acting district forest officer. Two years later he went to England for training and in 1895 was awarded the Certificate in Forestry of the Royal Indian Engineering College at Cooper's Hill, near London. Upon his return to South Africa he was appointed district forest officer at Butterworth in the Transkeian Conservancy from July 1896.
After the Anglo-Boer War (1899-1902) the British administration of the Orange River Colony (now the Free State), on the recommendation of J. Storr Lister*, conservator of forests in the Eastern Cape, appointed Carlson as the colony's first conservator of forests, in the Department of Agriculture, from 1December 1903. His brief was to investigate whether plantations of exotic tree species could be established in the colony. He immediately began the systematic trial introduction of trees and other plants from countries with similar climates and reported on some experiments with introduced species in his annual report for 1907/8. In 1908 he became a member of the South African Association for the Advancement of Science and also of the Philosophical Society of the Orange River Colony.
After the formation of the Union of South Africa in 1910 Carlson's post was changed to that of second grade conservator of forests - the most senior forestry post in the Free State - from 1 April 1912. The next year he published three articles in the Agricultural Journal of the Union of South Africa: 'Forestry in relation to irrigation in South Africa', 'Forestry in the Free State', and 'Windbreaks for the dry farm' (1913, Vol. 5). That year he succeeded Charles E. Legat* as conservator of forests in the Transvaal, a post he held until his retirement in 1923. In 1914 he atended the conference of foresters at the diamond jubilee of the Royal Scottish Aboricultural Society. His research in the Transvaal was described in two Bulletins of the Department of Forestry: The growing of mine props on the Highveld (No. 1, 1920, 12p) and Sylvicultural notes on Cedrela toona, with special reference to natural regeneration in Government plantation, Barberton (No. 4, 1921, 19p). A paper by him on 'Timber supplies and industrial progress' appeared in the South African Journal of Industries in 1922, and one on 'Timber consumption in South Africa' in the Empire Forestry Journal in 1926.
After his retirement Carlson retained a lively interest in forestry and in 1949, at the age of 86, was still vice-president of the South African Forestry Association. That year the King of Sweden awarded him a Knighthood of the Royal Order of Vasa, First Class. His autobiography, Transplanted: Being the adventures of a pioneer forester in South Africa, was published in 1947. He was a naturalized South African citizen, was married to Laura E. Dumbleton who died in 1918, and had one son.