He was wounded at the Siege of Savannah and was taken prisoner. He was exchanged after a few months and returned to Washington's staff. After the War, his first significant commission was the renovation of New York's City Hall into the First Congress of the United States. After the War, he built a successful architectural practice designing everything at all scales from medals (Society of the Cincinnatti) to houses for rich people to city plans to a tent for 1000 people for the Federal Procession of 1788.
Washington commissioned him to plan the new national Capital, a project which was extremely contentious. L'Enfant rose to the occasion and created a monumental axial scheme based on ended up getting fired and replaced by Andrew Ellicott whose version of the plan endured. The rest of his career didn't go very well. There was a mansion he designed for Robert Morris who managed to go bankrupt before it was completed, a conceptual plan for Hamilton's utopian industrial city of Paterson, NJ which took a year of work before he was invited to move on followed by several years as an Engineering Professor at West Point.