[3][4] He was educated at Shrewsbury School in England and graduated with a first class Bachelor of Arts degree in the mechanical sciences tripos, from the University of Cambridge in 1927.

[4] Jellett joined the drawing office of Rendel, Palmer and Tritton and spent the next three years designing railway girder bridges for the Ministry of Transport, principally in India and the colonies.

In 1935, he transferred to the Singapore Naval Base as deputy to the divisional officer in charge of construction of a new armaments depot with associated stores and workshops, on a reclaimed mangrove swamp.

Jellett was responsible for underground reinforced concrete magazines, sewerage, drainage, water supply, roads and a metre-gauge railway.

He was involved in planning dredging operations, construction of Fleet Air Arm stations and widening dry docks in HMNB Devonport and Gibraltar.

He was responsible for motor torpedo boat bases, minefield control towers, sea forts, shipyard and naval armament factories.

In 1942, he was superintendent civil engineer for the Eastern Mediterranean and carried out dredging and widening works on the Great Pass in Alexandria, renovating dry docks and constructing new slipways.

The citation in the London Gazette referred to his "distinguished service in operations which led to the successful landing of allied forces in Normandy".

His primary concerns at Southampton were with repairing war damage, reclaiming 450 acres of salt marsh and diverting the River Test.