Educated at Malvern and then Hertford College, Oxford, he joined the Artists’ Rifles in 1916, transferring to the RFC in early 1917, but was unable to go to France, due to his age, until February 1918.

On 30 October 1918, during a bomber escort mission to No 98 Squadron, the 12 Sopwith Dolphins of No 19 Squadron being led by Hardman and the DH9's of 98, were jumped by 30-40 Fokker DVII's.  In the ensuing combat all 'A' Flight aircraft were lost and Hardman and two other aircraft of 'B' Flight were the only ones to return to their base.  However, in the melee, Donald Hardman accounted for two of the German aircraft and was awarded the DFC for the action.  By the end of the war he had amassed a confirmed total of nine victories.

After WW1, he left the RAF and returned to Oxford but on completion of his studies, rejoined the RAF and was almost immediately sent out to 31 Squadron in India.

In 1939, he was selected by Group Capt Victor Goddard to join the Admin staff of the Air Component and accompanied Goddard to France in order to organise the movement of the various elements from the ports of entry on the Atlantic coast to their operating airfields in the North East of France.  Without their own 'works' staff, Hardman managed to commandeer a Sapper officer, who was able to mobilize local French contractors to carry out the simpler construction tasks required by the air force.  It was largely as a result of his efforts that the RAF arrived at their destinations ready for operations.

In 1956 he was appointed the President of the Board of Inquiry into the loss of Vulcan XA897 which crashed at Heathrow Airport followed a Round-the-World flight from which only the pilot and Air Marshal Sir Harry Broadhurst escaped.