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.