He was the eldest son of William Smelt (1690 – 14 September 1755) of Leases in Kirkby Fleetham, MP for Northallerton in the parliament of 1734. Leonard's first appointment was as a clerk in the Ordnance Office in June 1734 before becoming a cadet gunner in the new Royal Artillery on 1 January 1739. This did not take up all of his time and he was granted permission to attend the drawing room at the Tower of London, where he trained under Clement Lempriere and John Peter Desmaretz and became a skilled military artist and plan-maker. The Master General of the Ordnance, the Duke of Montagu, assigned him to Colonel Thomas Lascelles, chief engineer at Portsmouth Dockyard, in April 1741 and only two months later Lascelles recommended that Montagu grant him the rank of practitioner engineer. On 13 August 1741 he was given a post at the Tower of London by General John Armstrong, Surveyor-General of the Ordnance and Chief Royal Engineer. This post lasted almost a year before he was assigned to the British force in Flanders on 19 June the following year, becoming part of the ordnance train there and fighting at Dettingen and Fontenoy. He also gained promotion to engineer extraordinary on 8 March 1744, leapfrogging the intermediate rank of sub-engineer. In 1745 he was taken on by Captain Thomas, Royal Engineers, a subordinate of the Duke of Cumberland, and together they repaired and extended Vilvorden Castle's fortifications – they produced a plan of the new scheme which is now in the British Library.