Ducart's architectural practice can be divided into roughly two categories: his engineering projects at the collieries in Co. Tyrone, and his country house/public building work, mainly in Co. Limerick and Co. Cork. Ducart was first and foremost an engineer (with a particular expertise in hydraulics), but like other successful engineer/officer-architects who had worked in Ireland in the first half of the eighteenth century, such as Edward Lovett Pearce (qv) and Richard Castle (qv), he was able to build up a country-house practice. Ducart did not abandon his engineering work in favour of country-house commissions; rather he juggled projects and moved regularly between north-east Tyrone and parts of Munster, and was living in Dublin in 1769.
Ducart's involvement with the collieries in Tyrone began c.1765. In 1767 a quarter-mile (400 m) stretch of canal designed by Ducart was completed at a cost of £4,000. The linking of the collieries at Coalisland and Drumglass was one of the most ambitious and costly industrial projects undertaken in Ireland at the time, and it was not until 1777 that the communications (including a handsome aqueduct near Newmills) were completed. Ducart was the first engineer working in England or Ireland to have designed a system of inclined planes (rather than locks) on his canals.
His first documented building is the Custom House in Limerick, begun in 1765 and finished c.1769 at a cost of £8,000. In c.1769 he designed the street layout for Newtown Pery, the proposed development by Edmond Sexten Pery (qv) adjacent to the old city of Limerick. Ducart's grid plan was utilised by other architects when the town finally took shape in the early nineteenth century. Pery, a wealthy landowner and influential parliamentarian, was perhaps responsible for getting Ducart established in Ireland. While the Custom House was under way, Ducart was asked in 1765 to design the mayoralty house in Cork. Work began in 1766 and it was agreed that Ducart would receive 5 per cent of the estimated cost (£2,000). There were serious delays and the building was not completed until 1773. The civic officials criticised Ducart for his lack of attendance. From 1766 to 1770 he was busy working on a number of country houses in the locale, including Kilshannig and Coole Abbey and quite possibly the Island, Castle Hyde, Tivoli, and Dunkettle, all in Co. Cork. His most impressive surviving house is Castletown Cox, Co. Kilkenny, the seat of Michael Cox (qv), archbishop of Cashel (begun c.1767). His last documented house is Brockley Park, Queen's County (Laois) (finished 1768). In Co. Tyrone he designed an artificial lake and a bridge on the Lissan demesne (owned by his employer Thomas Staples) and probably added the wings and pavilions c.1770 to Florence Court.
Ducart became the leading country-house architect in Ireland after the death of Richard Castle (1751). He employed his own idiosyncratic Palladian–Italian baroque style, uninfluenced by the latest neo-classical currents. The basic layout of his houses is strictly neo-Palladian; the Custom House in Limerick and Castletown Cox, for instance (which consist of a central block and two wings), are like the farm complexes designed by Andrea Palladio. This form of Palladianism became less popular in Ireland after the 1750s, and Ducart (who used this style as late as 1770) has been described as the ‘last Palladian in Ireland’. Ducart's buildings were decorated with Italian baroque details that were more in keeping with buildings designed by Vanbrugh and Hawksmoor half a century earlier.
It is not known if Ducart married. He died in Ireland c.1785 and his will was proved on 29 March 1786. A photographic record of buildings attributed to Ducart is in the Irish Architectural Archive, Dublin.