Very little is known of Mongoose's past, before he came to New York City. He was smuggled out of his country of origin by the villainous Baron Zemo and his Masters of Evil. He was apparently supposed to join the Masters of Evil on their assault on the Avengers Mansion. The Masters' Absorbing Man and Titania, in disguise, were supposed to pick him up at the airport, but they became involved in a battle with the hero Spider-Man. Mongoose, now that he was in America, decided to forget about the Masters of Evil and set out on his own.
Mongoose was next seen attacking the hero Thor, who was at a construction site in his civilian identity of Sigurd Jarlson. Apparently the Mongoose had been hired by someone to kill Thor, for an unknown reason. The Mongoose, using his "cellsmograph," had tracked Thor down while he was in his secret identity. Spider-Man intervened, giving Jarlson enough time to get his hammer and turn into Thor.
Spider-Man and Thor fought Mongoose to a stand still, but Mongoose escaped after creating a diversion. Mongoose broke the support beams holding up the building where they were fighting. Fearing for the lives of the construction workers on the building, Thor held up the building long enough for Spider-Man to fashion some temporary support to avoid collapse.
The current location of Mongoose is unknown, though he is probably still somewhere in New York City.