During the Second World War in 1941 General Prannath Thapar was sent to Burma, Middle East, and Italy. In 1945 he was posted as the Assistant Military Secretary, General Headquarters in India. He was also made the member of the Army Reorganization Committee.
During the internal dilemma in Indonesia in 1946 General Prannath Thapar led the 1st Battalion Punjab Regiment in that country. He also led the 161 Indian Infantry Brigade in East Bengal (now Bangladesh). During the tumultuous period of partition General Thapar served as the Director of Military Operations and Intelligence at Army Headquarters. After that he rose to the rank of Major General and appointed as Chief of the General Staff and subsequently posted as Military Secretary. In 1949 he was given the post of Master General of Ordnance. Later he also commanded The Rajputana Rifles as the Colonel.
General Prannath Thapar led an Infantry Division for about four years and rose to the rank of Lt Gen in 1954.In 1955 he passed a course from the Imperial Defense College. After that he was served as General Officer Commanding-in-Chief in the Western Command follows General Officer Commanding-in-Chief in Southern Command.
General PN Thapar assumed the charge Chief of the Army Staff on 8th May 1961 and served till 19th Nov 62. He served 35 years to the nation. He was also a good tennis and golf player. General Prannath Thapar was sent as Indian Ambassador to Afghanistan in 1964. General Prannath Thapar left his mortal coil on 23rd Jan 1975 at the age of 69 years.