His son Franz Moritz von Lacy had entered the Austrian service in 1743 and became one of the most successful imperial commanders of the 18th century. Count Peter married Baltic German noblewoman Maret Philippine ("Martha") von Funcken from Liezere, widow of the young Count Hannes Kristof Frölich (d 1710), and daughter of general Remmert von Funcken, lord of Liezere, and his second wife baroness Helena Üksküla. They had 5 daughters and 2 sons. One son was Count Franz Moritz von Lacy, the famous Austrian Field Marshal who was also a Count of the Holy Roman Empire. Nothing is known about the other, supposedly older, son and in some references he is incorrectly attributed with accolades that belong to his father Count Peter and/or his brother Count Franz.