Pithie was commissioned in the 1st Otago Regiment in 1930, serving until 1936 when he was posted to the reserve of officers. Following the outbreak of World War II, he enlisted in the Royal Australian Air Force in Melbourne, but later returned to New Zealand where he joined the 2nd New Zealand Expeditionary Force. In April 1941, he was posted to the 7th Officer Cadet Training Unit, and was a platoon commander in the force sent to garrison Fiji that year. There he was promoted from lieutenant to captain, and he was appointed liaison officer for the brigade at divisional headquarters when he returned to New Zealand. He later served as adjutant of the advance party in New Caledonia, before attending Staff College in New Zealand. In September 1943, he rejoined the division at Vella Lavella as divisional intelligence officer. In December 1943, he was promoted to the rank of major and appointed brigade major to the 14th Brigade, serving in that role during the Battle of the Green Islands.