Mordecai was created in the genetic experiments performed by the Dominators when they attempted to take over Earth in the 30th century.
Mordecai is about 25 feet tall, and is immensely strong, agile and fast. His speech is slow, yet he seems to have some intelligence, and has written poetry.
Mordcai was living in the sewers under Metropolis, eating people, when he was discovered by Leviathan and Gossamer. Mordecai defeated them both and kidnapped Gossamer, who he seemed to fall in love with. Later the Legion of Super-Heroes defeated him and rescued Gossamer, but Mordecai escaped thanks to Leland McCauley and the Fatal Five.
Mordecai was chosen as a new member of the Fatal Five to replace Validus. Mordecai participated in an attack on the Earthgov Capitol orchestrated by Leland McCauley, but was defeated along with the rest of the Fatal Five by the Legionnaires.
Mordecai's co-creator Tom Bierbaum had this to say of the character on his blog: "My idea with Mordecai was to make him a self-pitying brute. He viewed himself as a victim of all sort of cruelties while at the same time feeling no sympathy for the victims of the violence and death he was dealing, which he always felt was justified. He longed for the attentions of an attractive female, hence his attempt to turn Gossamer into his own little bird in a gilded cage in "Legionnaires #3". He seemed to see himself as the tortured hero in kind of a “Beauty and the Beast” story, though Gossamer somehow proved quite impervious to his charms."