Cecil N. Barker requested compensation from the British Colonial Office in 1879 for a wound, which probably resulted from military service during the Anglo-Zulu War that year. In 1890 he reported to the Colonial Secretary's Office in Natal on his success with the hatching and rearing of silk worms. By 1893 he lived in Malvern, Durban, where he appears to have remained all his life. He was an ardent student and colector of Natal insects. He started collecting before 1893, for in that year is said to have continued his donations of rare lepidoptera, with natural history notes, to the South African Museum. He also provided the museum with a considerable series of coleoptera this year, including 32 species that were new to its collection. The specimens had been collected in Natal at Karkloof, Malvern, Estcourt, and the Black Umfolozi. His observations relating to butterflies were written up as "Notes on seasonal dimorphism of Rhopalocera in Natal" and published in the Transactions of the Entomological Society of London (1895).
From about 1894 Barker concentrated most of his attention on the coleoptera (beetles), presenting the South African Museum with specimens each year. In 1898 he consulted the museum's insect collection and donated 892 specimens, representing 280 species of which 44 were new to the museum. Most were coleoptera, but some species of hymenoptera were also included. One reason for his success was that he collected near electric lights in Durban, which attracted many types not yet known to occur there. In 1899 he contributed 327 species of coleoptera, of which 9 were new to science, as well as two new species of hymenoptera. In recognition of his consistent support he was named as one of only nine "correspondents" of the museum this year, which meant that he would receive its publications free of charge. Barker continued his donations during 1900-1904. In the latter year he sent a collection of chrysomelidae (leaf beetles), which were described by the British entomologist M. Jacoby. In 1908 his consignment of coleoptera from Natal contained 16 species new to the museum. More followed in 1909, this time mainly carabidae (ground beetles).
In 1916 Barker was on friendly terms with E.C. Chubb*, then director of the Durban Natural History Museum, and offered to sell his personal collection of coleoptera to that museum. The collection comprised 11 436 specimens, representing 4 141 species, and Janse (1919) considered it the third most important collection of coleoptera in the country after the museum collections in Cape Town and Pretoria. The museum council approved the deal in March 1916, but could pay the 600 pounds sterling agreed upon only in three annual installments. Meanwhile Barker worked for the museum without salary from 1916, but in 1919 was officially appointed as scientific assistant. During these years he studied the collections of various insect groups and published the results in six papers in the Annals of the Durban Museum. The first paper dealt with melanic aberrations in butterflies collected for the museum by A.D. and H.M. Millar* (Vol. 1, 1914-1917, pp. 451-457), while the rest all appeared in Vol. 2 (1917-1920) of the Annals. Probably his most important publication was "A criticism of the foundation upon which the theory of mimicry is built", which appeared in the South African Journal of Natural History (1919, Vol. 1, No. 2, pp.138-163). In this review he questioned in particular the assumption that discriminative bird predation is intense enough to drive the evolution of butterflies. His main conclusion was that "the evidence in support of mimicry by natural selection as a protective agency is quite insufficient and often misleading". He also described new South African species of the family Carabidae (ground beetles) in three papers published in the (British) Journal of Natural History, all three in 1922.
Barker left the Durban Natural History Museum in 1922 when its government aid was withdrawn, but continued his study of its insect collections. He became a member of the South African Philosophical Society in 1897 and remained a member when it became the Royal Society of South Africa in 1908. He was a Fellow of the Entomological Society of London, and became a foundation member of the South African Biological Society in 1916.