Born on 13 April 1901 in Tylden, Victoria, Joseph Eric Hewitt was the son of Joseph Henry Hewitt and his wife Rose Alice, née Harkness. He attended Scotch College, Melbourne, before entering the Royal Australian Naval College at Jervis Bay in 1915, aged 13. After graduating in 1918, Hewitt was posted to Britain as a midshipmen to serve with the Royal Navy. He rose to lieutenant in the RAN before volunteering for secondment to the Royal Australian Air Force (RAAF) as a flight lieutenant in January 1923. Hewitt undertook the pilots' course at No. 1 Flying Training School, Point Cook, and graduated at the end of the year. He was further seconded to the Royal Air Force in May 1925, holding a temporary commission as a flying officer until September. He married Lorna Bishop in Sydney on 10 November; they had three daughters.
Seagull III of No. 101 Flight being hoisted aboard the seaplane carrier HMAS Albatross
In August 1926, Hewitt joined the newly formed No. 101 (Fleet Cooperation) Flight, operating Seagull III amphibians. Prior to the unit deploying to Queensland to survey the Great Barrier Reef with HMAS Moresby, he practiced manoeuvres around the centre of Melbourne, landing in the Yarra River near Flinders Street station. Media criticism of the escapade led to him being brought before the Chief of the Air Staff, Group Captain Richard Williams, who rather than upbraiding Hewitt expressed himself "reservedly pleased about the publicity". After completing its survey work in November 1928, the unit served aboard the seaplane carrier HMAS Albatross.
Hewitt's transfer to the Air Force was made permanent in April 1928. Promoted to squadron leader, he became commanding officer of No. 101 Flight in February 1931, and supervised embarkation of the Seagull aboard the cruiser HMAS Australia in September–October 1932. Hewitt finished his tour with No. 101 Flight the following year, and was posted to Britain in 1934. He attended RAF Staff College, Andover, in his first year abroad, and served as Assistant Liaison Officer at Australia House, London, in 1935. Although a specialist seaplane pilot, he converted to bombers in England, flying Hawker Hinds and Bristol Blenheims as commanding officer of No. 104 Squadron RAF from 1936.
Hewitt was promoted wing commander in January 1938. Returning to Australia, he was appointed senior air staff officer (SASO) at RAAF Station Richmond, New South Wales, in June. In May 1939, Hewitt was chosen to lead No. 10 Squadron, due to be formed on 1 July at the recently established RAAF Station Rathmines, near Lake Macquarie. He was preparing to depart for England to take delivery of the unit's planned complement of Short Sunderland flying boats when he broke his neck riding his motor cycle near Richmond, and had to forgo the assignment while he recovered. Fit for duty by August, he was given command of the Rathmines base to manage the deployment of No. 10 Squadron and its aircraft, but this was suspended due to the outbreak of World War II in September, and the Sunderlands and their RAAF crews remained in Britain for service alongside the RAF.