John of Horne is best known for his role in the 1436 Siege of Calais. Philip the Good besieged the English-held city and as Admiral of Flanders, it was John's task to block the harbor of Calais. He sank 5 to 6 ships laden with stone in the harbor entrance, but the people of Calais were able to approach these wrecks during low tide and dismantled them. Furthermore, John was forced to return his fleet to Holland because of bad weather. His failure to block the harbor contributed to the overall failure of the siege.