He completed a three-year tour with the King's African Rifles in East Africa and, in 1939, returned to the 1st Battalion, Green Howards in Palestine as Intelligence Officer.

At the outbreak of World War II, he was an Air Liaison Officer in East Africa before working on the staff for the Greek Campaign between 1941–42.

When he was 39 years old, Derek Seagrim was made a temporary lieutenant colonel in the 7th Battalion, The Green Howards (Alexandra Princess of Wales's Own Yorkshire Regiment).

On 20/21 March 1943 at the Mareth Line, Tunisia, Lieutenant Colonel Seagrim's courage and leadership led directly to the capture of an important objective.

When it appeared that the attack on the position would fail owing to the intensity of enemy fire, he placed himself at the head of his battalion and led them forward.