James Chalmers, son of Henry Reid Chalmers and his wife Georgina, born Reid, studied at the Royal Veterinary College, London, from 1896 and qualified as a member of the Royal College of Veterinary Surgeons (MRCVS) in 1900. In 1901 he came to South Africa as a civil veterinary surgeon attached to the Army Veterinary Department and during the remainder of the Anglo-Boer War (1899-1902) served first on general French's staff and then in the Repatriation Department. He joined the public service of the Transvaal Colony in March 1903 as a government veterinary officer in the Department of Agriculture.
In March 1906, when he was a district veterinary surgeon in Heidelberg, Transvaal, he read a paper on "Acariasis, scab, or mange of sheep" at the 12th quarterly meeting of the Transvaal Veterinary Medical Association in Pretoria. The paper was subsequently published in the Transvaal Agricultural Journal (Vol. 4, pp. 795-803). He still resided in Heidelberg in 1908, but in 1909 became government veterinary surgeon for the Witwatersrand. Later he served as external examiner for the BVSc degree of the Transvaal University College, the degrees at that time being awarded by the University of South Africa.
Chalmers joined the Transvaal Veterinary Medical Association in 1903, the year of its formation, served as its honorary secretary in 1907/8 and 1908/9, as a member of its council in 1909/10, and as vice president in 1910/11. In 1919 he joined the newly established South African Veterinary Medical Association and was still a member in 1929. In 1917 he became a foundation member of the South African Biological Society. At this time he was stationed in Piet Retief, Transvaal. He remained in government service to his retirement in December 1930. Thereafter he took up private practice in Johannesburg, while at the same time serving as veterinary officer to the South African Institute of Medical Research, as well as quarantine officer for the territories that are now Botswana, Swaziland, Lesotho, and Zimbabwe. He was married to Mabel Bessie Chalmers, born Russell, with whom he had a son and a daughter.