James Davidson, adjutant general of Texas, was born in Kelso, Roxburgh County, Scotland, in 1834. He received a common-school education and was proficient in reading and writing. When he immigrated to the United States in 1865 he claimed that after leaving school he had served in a cavalry regiment in the British army. In 1865 he settled in Maine. In March of that year he enlisted as a second lieutenant in an unassigned company of the Nineteenth Maine Infantry Volunteers. He was mustered out on May 23, 1865, at which time his commander described him as "zealous, energetic and competent." He joined the regular army the following June as a private in Company D, Corps of Engineers. He remained with the engineers until June 1867 and rose to the rank of first sergeant. In May 1867 he passed examinations allowing him to be appointed second lieutenant and was assigned to the Twenty-ninth Infantry. He transferred to the Eleventh Infantry on April 25, 1869.
In 1869 the Eleventh Infantry was headquartered at Jefferson, Texas. Davidson arrived in Clarksville and assumed the duties of military commissioner for Red River County on June 18. In this position he achieved a reputation for strict enforcement of military rule. He restricted the wearing of sixshooters and other weapons in town. He actively supported efforts by the local Freedmen's Bureau agent to educate and protect freedmen. During his service in northeastern Texas he became associated with prominent Republican politicians Amos Morrill and Albert H. Latimer.
Governor Edmund J. Davis nominated Davidson adjutant general of Texas with the rank of colonel on June 24, 1870. The Senate confirmed the nomination on July 6, and Davidson resigned from the United States Army with an honorable discharge. As adjutant general, Davidson was chief of the State Police, which he organized quickly. He also headed the state militia and was promoted to major general of it on March 15, 1871. His early priority in office was the suppression of outlaws and the Ku Klux Klan. He aggressively used the police and the militia in his efforts and personally headed militia operations following declarations of martial law in Madison, Hill, Walker, Limestone, and Freestone counties in 1870 and 1871. Under his leadership the State Police forcefully hunted down criminals and made 978 arrests in the first month of the organization's existence.
The State Police and the militia both were politically controversial institutions. Democrats charged that they were instruments of Republican tyranny. The fact that many members of both the police and militia were Black further irritated Whites. While both the militia and police received criticism, Davidson generally avoided personal attacks. In September 1871 a disgruntled state policeman accused him of malfeasance in procuring supplies for the police. Though the incident received attention in Democratic newspapers, the charges were never proved. Even among his political opponents Davidson earned a reputation for honesty and efficiency in operating the police and militia. When he readily complied with conservative efforts to cut back and reorganize the State Police after the spring of 1872 he received Democratic praise for being one of the better members of the Davis government.
On October 12, 1872, the local Republican executive committee at Austin nominated Davidson for state senator from the district embracing Travis County. He resigned his position as adjutant general on November 4 to run for the office on a platform that declared support for law and order and public education. He also broke with the Davis administration when he pledged to uphold the legislature's grant of state bonds to the International-Great Northern Railroad, a position opposed by the governor. In the election he faced Nathan G. Shelley, and although he received some praise in Democratic newspapers he still lost, in an election marred with charges of fraud. In December after the 1872 election, Davidson accompanied secretary of state James P. Newcomb to New York City, where Newcomb had gone to sell state bonds. In his absence, the new adjutant general, Frank L. Britton, discovered shortages in Davidson's accounts. Davidson had drawn warrants on the treasury for payment of State Police officers, which the comptroller allowed without vouchers filed showing legal services, and apparently pocketed the funds. The state immediately ceased payment on the warrants and attached Davidson's property to limit the potential loss. Later investigations placed the loss at $37,434.67, although what was recovered by the seizure of Davidson's property is not known. Governor Davis offered a reward for the former adjutant general's return and sent Capt. Leander H. McNelly of the State Police in pursuit. McNelly reported that Davidson probably had left the United States, although the policeman never determined the fugitive's location. He was later reported to have fled to Belgium but was not found there and never returned to Texas.
Davidson surfaced in New Plymouth, New Zealand, where he married Jane Ryan on October 20, 1874. They had six children. Apparently he was a member of the Armed Constabulatory Force in New Zealand. By 1880 he was a captain in the Taranaki Mounted Rifles headquartered in New Plymouth, and he served as mayor of that town from 1879 into the 1880s. Davidson committed suicide on April 7, 1885. His death records listed his profession as an accountant. He was buried in New Plymouth, New Zealand.