[1][4] Whilst a student at Edinburgh, Thapar organised the Scottish Wing of the Indian Volunteers' Ambulance Corps following the outbreak of war in 1914.
Holding the rank of volunteer sergeant-major, he recruited 120 Indian students into the corps, which provided dressers and interpreters to army medical units.
[8][9] Thapar was promoted to major on 25 September 1929,[10] and was appointed a staff captain (medical) on 1 July 1936.
[12] On 14 October 1937, Thapar was posted to Peshawar as second-in-command of a hospital, and was appointed as the officiating Officer Commanding on 3 May 1938.
[14] Assigned as Officer Commanding Indian Hospital Corps at Kirkee on 22 October 1942,[15] Thapar was promoted to acting colonel on 5 November 1942 and to temporary colonel on 5 May 1943,[16] while serving as Commandant, Indian Army Medical Corps (IAMC) HQ at Pune.