Christiaan Pieter van der Merwe was educated at Grey College, Bloemfontein. During the Anglo-Boer War (1899-1902) he did military service and was sent to India as a prisoner of war. After the war he entered the civil service of the newly created Orange River Colony (now the Free State) in March 1903. In 1906 the government of the colony awarded him a scholarship to enable him to study at the Hawksbury Agricultural College in New South Wales, Australia. There he obtained a Diploma in Agriculture, though his main interest was in entomology. Upon his return to South Africa in March 1908 he was appointed as assistant agronomist in the Division of Horticulture and Biology, Department of Agriculture, of the Orange River Colony in Bloemfontein. At the beginning of 1909 he succeeded Harry Neethling* as head of the division. Shortly after the formation of the Union of South Africa in 1910 he was appointed entomologist in the Division of Entomology, Department of Agriculture of the Union, stationed in the Free State. Around this time he wrote two articles for the Agricultural Journal of the Union of South Africa, one on "Varieties of apples suitable for the Orange Free State" (1911), the other on "The Codling Moth" (1912). However, by 1914 he was stationed in Durban, where he had a small office at the Point and among others was one of the government scientists supervising the planting of new varieties of sugar cane at an experimental station managed by the Natal Sugar Association. In 1921 he moved his office to a room in the Natal Herbarium and three years later was still doing pest control work in Durban for the Division of Entomology.
His work with sugar cane enabled van der Merwe to gather, for the first time in South Africa, information on sugar cane insects, which he later published. His scientific publications also included "A note on Dasychira extorta and its lepidopterous parasite" (South African Journal of Science, 1920, Vol. 17, pp. 192-194) and "Aphis leguminosae - A biological study" (Entomology Memoirs, 1931, No. 7, pp. 5-16). Between 1916 and 1920 he presented ants to the South African Museum, and fungi that he collected are now in the National Collection of Fungi of the Plant Protection Research Institute in Pretoria. He was the first person to translate a list of entomological terms into Afrikaans. The list was publised in Die Huisgenoot around 1924.
In 1927 van der Merwe became head of the Division of Plant Regulatory Services (which included mursery inspection and port fruit inspection) and settled permanently in Pretoria. He held this post until his retirement in 1940, but continued his services until 1952, when he became 72 years old.
Van der Merwe was a member of the South African Association for the Advancement of Science by 1910. He became a foundation member of the South African Biological Society in 1916 and regularly attended its meetings. During 1933-1936 he was a member of the Pretoria Entomological Club, and in 1937 became a foundation member of the Entomological Society of Southern Africa.