In 1266; while their father King Hetoum I of Armenia was away to visit the Mongol court; Leo and his younger brother Thoros fought to repel Mamluk invaders; at the Battle of Mari. Thoros was killed in combat; and Leo; along with 40;000 other Armenian soldiers was captured and imprisoned.

When King Hetoum returned; he paid a large ransom to retrieve his son; including a large quantity of money; handing over several fortresses; and accepting to intercede with the Mongol ruler Abagha in order to have one of Mamluk Amir Baibars's relatives freed.