Lethbridge was commissioned into the Corps of Royal Engineers in 1915 during the First World War and saw active service on the Western Front with 123 Field Company at the first battle of Ypres in 1915 and later at the Battle of the Somme in 1916 and as a temporary Captain with the King George V's Own Bengal Sappers and Miners in India in 1917 before serving with the Aden Frontier Force for operations in southern Arabia between 1917 and 1918. He commanded, as acting major, a company of King George V's Own Bengal Sappers and Miners in Afghanistan and on the North West Frontier during the Third Anglo-Afghan War from 1919 to 1922. He served again with King George V's Own Bengal Sappers and Miners at Roorkee and Rawalpindi and saw active service in Peshawar against Afridi raiders in 1930. After graduating from Staff College at Quetta in 1932 he became Superintendent of Instruction at Roorkee and was then appointed a Field Works major at Chatham in 1933. He joined the General Staff at Headquarters, Northern Command at York and then transferred to the Military Operations Branch and Directorate of Recruiting and Organisation at the War Office in 1936 before becoming an instructor at the Senior Officers' School at Sheerness in 1939.