Both combatants on Lake Ontario were building progressively larger ships of war. Yeo laid down a frigate, to be named "Prince Regent", and requested the Admiralty to promote Mulcaster to the rank of Post-Captain to command her. This was agreed. Before the ship was launched, in late 1813, Mulcaster commanded several vessels from the Lake Ontario flotilla which were escorting supply convoys up the Saint Lawrence River. He encountered the vessels of American General James Wilkinson's expedition against Montreal at French Creek and harassed the American encampments. When Wilkinson set off down the Saint Lawrence, Mulcaster hastened to Kingston with the news and then sailed in pursuit with armed schooners and gunboats, with a detachment of soldiers commanded by Lieutenant Colonel Joseph Wanton Morrison embarked. On 11 November Mulcaster's gunboats helped goad Wilkinson into a hasty attack against Morrison which led to the American defeat at the Battle of Crysler's Farm.