In Mega Man & Bass, King created six robots to help him and had help from two of Dr. Wily's robots, Astro Man and Tengu Man. He attacked the Robot Museum and stole the data of the battle robots to create a robot army. Proto Man tried to stop him, but he was defeated in one hit. King left the Green Devil to cover his escape. King is later on fought at the second King Castle stage as the penultimate boss. At first, he seemed unbeatable since he had a reflective shield that would send Mega Man's and/or Bass' shots right back to them. However, the newly repaired Proto Man warps in and destroys his shield with his Big-Bang Strike attack, leaving King vulnerable. After that, Mega Man and/or Bass were able to engage King in combat once more and defeat him.
Defeated and weakened, King asks the heroes why they would go so far as to fight for "lowly beings" like humans, and tells them that robots are more advanced. Mega Man responds by telling him that neither of them are better or more advanced than the other, while Bass, on the other hand, tells him not to forget that humans are the ones who created robots in the first place. After that, King tells the heroes to finish him off, but not before warping the exhausted Proto Man out of the area. Mega Man tells King to escape from the castle with him, and that Dr. Light will be happy to repair him. King responds by telling Mega Man that it won't work, because his body is set so that it'll explode if anyone other than Dr. Wily tries to tamper with it. In Bass' story, he'll ask King who his creator is, to which he answers Wily.
In both stories, Dr. Wily will appear on the monitor, scolding King for siding with the enemy, as well as telling him to finish the two heroes off. King ignores him and instead asks again why robots fight for humans. Then, Dr. Wily uses a device to increase King's brainwashing (and supposedly, his power level as well). After that, King flees into the next room, and then Mega Man and/or Bass will chase him. After that, King merges with a blue Mechaniloid-esque robot that was fought two other times, and engages the heroes in combat once more. After King is defeated again, he changes back to normal, telling the heroes to get out of there quickly before the castle explodes. First, they ask about him, but he responds by saying that he'll make it out of there after they do.
In Bass' story, King says that if he was ever reborn, then he and Bass could become good friends. In Mega Man's story, he'll say not to let anymore robots like him be made. After King's "last" words, the castle exploded, burying him in rubble in the process. After Mega Man returned home saddened from his battle, Roll gives him a letter with King's insignia on it (meaning that it was obviously from him, which confirms that he survived), and it turns out to be a letter thanking Mega Man for helping him realize the error of his ways. He then says that if they are to meet again, he hopes that it'll be as friends and not as enemies. In Bass' story, Dr. Wily revealed plans for a King II, offering Bass a chance to team up with him when he is complete. He then tells Bass that if he joins with King II, then the two of them will be an unstoppable duo. Before Bass can answer, however, Proto Man warps in and destroys the blueprints, foiling Dr. Wily's plans once more.