In 1266, while their father king Hetoum I was away to visit the Mongol court, Leo and his younger brother Thoros fought to repel a massive army of Mamluk invaders, at the Battle of Mari.

In the past, men were courageous at war, but today they are vile and chetive, and don't have other talents than drink properly.

The following year, Armenia fought off an invasion by the Turkoman tribes, but the Constable Sempad, Leo's uncle, was killed in combat.

Leo had to sue for peace, and in 1285 obtained a 10-year truce in exchange for important territorial concessions in favour of the Mamluks.

During twenty-one years of marriage Leo had sixteen children by his wife Keran, ten sons and six daughters.

Eventually, the inheritance of the Armenian Kingdom was passed to the descendants of Leo and Keran's eldest surviving daughter Isabella of Armenia, wife of Amalric of Lusignan, Lord of Tyre.