As an adult, Merrano worked as a research scientist in the Atlanteans' Antarctic city.  However, an insatiable lust for power grew within him. In a desperate grasp for power, Merrano allied himself with Nazi Germany.  He betrayed the Atlantean city's location to the Nazis, who quickly launched an attack, severely damaging the Atlantean fleet, destroying the Atlantean Hall of Science, and leaving Atlantis's emperor in a coma.  Acting as emperor, Namor banished Merrano for treason.

Exiled from his homeland, Merrano wholeheartedly joined the Nazis in their insane effort toward world conquest.  Merrano believed a Nazi victory would bring him enough personal power to fulfill his dream of revenge against Prince Namor.

Combining his own advanced Atlantean scientific knowledge with that of the Nazis, Merrano invented a way to increase his size and strength to the point where he rivaled the Sub-Mariner.  Taking the name U-Man, he became the living equivalent of one of Germany's devastating U-boats.

U-Man often came into conflict with Namor and Namor's allies, the small strike force of adventurers called the Invaders.  On one such occasion, U-Man took command of a fleet of Nazi U-boats with orders to intercept and destroy Prime Minister Winston Churchill's flying boat, the Berwick, which was due to land for a rendezvous with the H.M.S. Duke of York, a British battleship.  Fortunately, just as the flying boat was about to touch down on the waves, it was warned and began to climb into the air.  However, the Berwick's pontoons skimmed the water, giving U-Man just enough time to grab hold.  Peering from a window, the Prime Minister watched in dismay as U-Man started to tear off a wing of the plane.  Luckily, the Sub-Mariner arrived at that moment.  After a brief battle on the wing of the Berwick, the two Atlanteans fell into the sea.

Namor had nearly defeated U-Man, but he was forced to leave the villain and go to the aid of Winston Churchill's plane, which had been caught in a Nazi weapon that produced a spiraling headwind.  With the help of the other Invaders, the Sub-Mariner freed the Berwick from the strange weapon and brought it in for a safe landing.  U-Man and his men managed to escape during the confusion.

Later during the war, U-Man was inexplicably drawn from his watery domain to the streets of Manhattan's Chinatown, and a place called the "House of Lotus."  An enigmatic Japanese woman called Lady Lotus, using her psychic abilities, had brought U-Man to her, and now controlled him. Merrano did not resist, and became a willing ally of the Japanese spy.  Lady Lotus also brought Baron Blood, Master Man, and Warrior Woman under her dominion.  After forming this unholy alliance, Lady Lotus named the group the Super-Axis.

For their baptismal assignment, Lady Lotus sent the Super-Axis to Chicago.  Using her psychic powers to speak directly to their minds, Lady Lotus commanded the Super-Axis to wreak havoc at the largest railroad terminal in the country.

Everything was going as planned until Captain America, the Human Torch, Miss America, the Whizzer, and the Sub-Mariner arrived on the scene.  From her hideout in Chinatown, Lady Lotus watched the battle rage within her crystal ball.  Fearing the National Guard would soon arrive, she ordered the Super-Axis to retreat.

The Invaders followed the Super-Axis to Riverview, a giant amusement park in Chicago.  Lady Lotus ordered the villains to destroy the rides, in the hope that they would kill American servicemen on leave there.  As U-Man went about dismantling the support structure of a roller coaster, he was ambushed by the Sub-Mariner and knocked into a huge, lighted sign.  The electricity rendered U-Man unconscious.  The Invaders successfully kept any civilians from being killed, and rounded up the rest of the Super-Axis.  Lady Lotus, however, was not captured.

The Invaders would come to defeat U-Man time and again.  The final end to Merrano's plans for vengeance came with the defeat of Germany in 1945.  Unfortunately, U-Man was never captured and tried as a war criminal.

Recently, U-Man aided the Atlantean warlord Attuma in his failed attempt to conquer the surface world.  Later, U-Man, Tyrak, and Orca, along with a number of Atlantean warriors, intercepted and attacked a large group of heroes who had "invaded" their underwater realm.  A Soviet nuclear submarine, named the Waterwind, had been taken over by terrorists.  Five members of the Avengers (Captain America, Vision, Sersi, Quasar, and Stingray) responded to the danger, as well as five members of the Soviet People's Protectorate (Red Guardian, Vostok, Fantasma, Crimson Dynamo, and Perun).  The two hero groups arrived at the submarine at the same time, and battled one another.  However, they soon joined forces when the enraged Atlanteans attacked.

During the fierce battle, a stray shot fired from U-Man's weapon hit the submarine, forcing it to surface.  The terrorists docked at St. John's, Newfoundland.  Meanwhile, the combined might of the Avengers and the People's Protectorate forced the Atlanteans to retreat.

The leader of the terrorist group, known as the Peace Corpse, demanded access to the island's facilities in order to repair the sub, or they would execute one hostage every five minutes.  Taking charge, Captain America refused to negotiate, and the terrorist shot Stingray (who had been captured), apparently killing him.  At that moment, five members of Alpha Flight (Guardian, Shaman, Box, Puck, and Diamond Lil) arrived and took charge, claiming that the Canadian government had authorized control of the situation to them.  Captain America and Red Guardian reluctantly agreed to let Guardian take charge.

In order to buy time, Guardian agreed to let the terrorists begin repairs on the sub.  In the meantime, the three groups of heroes began to develop a battle plan.  They successfully recaptured the ship, but upon entering the missile control room, they found that two of the terrorists had wired themselves directly to the missile launch system.  Just as the heroes were deciding what to do, U-Man and the Atlanteans attacked once again.  During the mayhem, the submarine's warheads detonated, sending all of the heroes, villains, and citizens of the small island to an alternate dimension.  After a long ordeal in the alien realm, all of them were returned safely to Earth.  Merrano's present whereabouts and activities are unknown.