Mar-Vell’s regiment, under the command of Yon-Rogg, was given a mission to observe Earth, specifically human’s progress of space travel. Yon-Rogg, jealous of the love between Mar-Vell and the ship’s medic, Una, ordered Mar-Vell to infiltrate Cape Canaveral, Florida, alone. He then secretly tried to fire on Mar-Vell’s ship, but killed a human instead-- Dr. Walter Lawson. Mar-Vell took advantage of resembling Lawson to pose as the doctor and create a secret identity, but Yon-Rogg again secretly tried to kill Mar-Vell by activating the Kree robot, the Sentry. In his Kree uniform, Mar-Vell publicly defeated the robot, and the onlookers acclaimed him as "Captain Marvel," misinterpreting what the Sentry called him.
Mar-Vell continued to observe humanity, in both his secret and costumed identity, growing increasingly sympathetic for Earth and its inhabitants. His initial adventures were typically extensions of Yon-Rogg’s attempts to kill, discredit, or otherwise destroy him, such as the time Yon-Rogg manipulated Ronan the Accuser to fight Mar-Vell by framing Captain Marvel as a traitor. Mar-Vell also had to navigate the romantic triangle between him and Una and the aggressive attention of his co-worker at the air base, Carol Danvers. These situations came to a head when Una overheard one of Yon-Rogg’s plans to kill Mar-Vell, and she was captured by his agents when she tried to warn him. Although Mar-Vell’s execution failed with the arrival of the Aakon aliens, she was shot in the battle and died in Mar-Vell’s arms as he tried to rescue her.