Soon afterwards, Asgill was ordered to North America to fight in the American Revolutionary War. He shipped out for America in March 1781. After Asgill joined Cornwallis's army, his company commander fell ill. The young lieutenant and captain Asgill took charge of the unit and led it in a successful attack on a post held by local militia under an elderly colonel named Gregory. Colonel Gregory was wounded and captured, and Asgill won the admiration of his enemies for his kindness toward a fallen foe. Roger Lamb, writing in 1809, quoted an extract from a "Hibernian Magazine" article of 1782, in which he wrote "may serve to shew what was the prevailing opinion of the day relative to that officer":