James Thacker qualified as a veterinarian (MRCVS) in London in January 1844 and from 1851 to 1853 served with the British Army in the Cape Colony. In 1853 he was stationed as veterinarian to the Cape Mounted Riflemen at King William's Town, at the time when the 12th Lancers handed over their horses before departing for service in India. In October that year he described four cases of disease in horses, two were probably glanders, the others probably phrenitis and enteritis with rupture of the colon. His paper was published in the Veterinarian in 1854, making him the first veterinarian in South Africa known to have contributed an article to veterinary literature.
Thacker was a very good rider but was reported to have broken every bone in his body. As a result of these injuries he rode with a cane, which caused him to lose the sight in one eye in an accident. He visited the Cape again in 1875 as a remount officer. After retiring from the army that same year he was appointed remount agent in Bengal (now part of both India and Bangladesh). He died during a visit to Australia to purchase remounts. In his obituary in the Veterinarian he was wrongly named William Thacker.