A hero published by Marvel Comics. Jack of Hearts first appeared in Deadly Hands of Kung Fu #22 in 1976.

Jack Hart was the son of Philip and Marie Hart and lived a relatively normal childhood. Philip Hart was a scientist who was working on a alternate energy source called Zero Fluid that appeared to be a cheap and efficient way to provide energy. Philip planned to give Zero Fluid to the world to help with the then present energy crisis.

The elder Hart's work in alternate energy sources had brought him to the attention of two groups who were also interested in his work. The first was a group of aliens named the Contraxians, whose star was waning and who hoped to discover a way to reenergize it. To this end, the Contraxians sent agents throughout the universe to seek alternate energy sources. One of these agents was Marie Har, who had fallen in love with and married Philip Hart while spying on him. This made their son, Jack, a half-human/half-Contraxian. She died in an accident before Hart completed his work.

The other group who was interested in Hart's work was a shadowy group called the Corporation. The Corporation's energy interests were threatened by Hart's work and once he had perfected the Zero Fluid, agents of the Corporation attempted to steal it and the formula for it from Hart. Philip Hart was killed by agents of the Corporation and his son Jack escaped, but not before being doused with the only sample of Zero Fluid. In a rather Harvey Dent coincidence, Jack was only doused on the left side of his body which was burned black and the the fluid caused Jack's unique physiology to mutate so that he began to release large amounts of energy. He returned to his father's lab and fashioned for himself from spare parts around the lab a suit of armor that helped him to regulate his energy output. Vowing vengeance upon the Corporation, Jack combined his new armor with other costume parts and created an identity so visually complicated as to make artists weep. And so the Jack of Hearts was born as a tribute to his father's love of cards and play on words of his own name, showing once again the danger of names in a super-hero reality.

Jack of Hearts fought against the Corporation and teamed up with both the White Tiger and Hulk during his early career. He briefly came under the tutelage of Iron Man during a period when Stark International was battling a takeover by the evil industrialist Mordecai Midas. Iron Man helped Jack to learn to use his new powers, and during this time, Jack was fitted with a Stark invention called the Scanalyzer that allowed him to think and analyze at computer speeds.

After parting company with Iron Man, Jack found his powers growing ever more powerful and he had less and less control of them. Fearing he would be a danger to the public, Jack turned himself over to S.H.I.E.L.D., who employed a mist to help regulate his powers. During this time, Jack met and fell in love with a scientist named Marcy Kane. Kane revealed to Jack that she was from Contraxia and helped him to piece together parts of his heritage. Jack went with Kane to Contraxia in a bid to regenerate their star. Jack used his enormous power to restart the star, but when he returned to Contraxia, his power output had greatly increased melting the ice caps of Contraxia and putting the population in danger. Jack left for the stars, thinking himself incapable of being near other people from that point forward.

For a time, Jack wandered the universe until he was captured by the Elder of the Universe, the Collector and taken to the planet that housed his collection. Jack was freed by the actions of Quasar and returned to Earth. Once there, he was the victim of mind-control by a villain known at the Presence and forced to fight Quasar. During the battle, Jack of Hearts was seemingly killed when his armor was breached, but he was rescued by Moondragon, a former Avenger. For a brief time, Jack was used by Moondragon in an attempt to gain favor with Quasar, but eventually, Moondragon gave up and Jack was freed.

Still in danger of losing control of his power, Jack sought the help of the Silver Surfer. Silver Surfer guided Jack to an alien who constructed Jack a superior set of armor. Jack then helped the Surfer and others in fighting the threat of the Tyrant. Though they were successful in their attempt to end the Tyrant's plans, Jack was injured and traveled for a time with an alien female named Ganymede. They began a relationship which eventually ended and Jack returned to Earth.

On his journey to Earth, Jack encountered a group of beings known at the Infinites and was nearly killed by the encounter. Jack was rescued by the Avengers who restored him to his original armor design but with superior abilities. Jack helped the Avengers fight the Infinites and later aided them when they turned back an attempt to turn the Earth into a interstellar penal colony.

Upon their return to Earth, the Avengers offered Jack membership in their team which he accepted and is actively serving as a member of the team currently.