Metallo, most commonly known through the identity of John Corben, first appeared in Action Comics #252 (May 1959), created by writer Robert Bernstein and artist Al Plastino. In his original Silver Age incarnation, Corben was a unscrupulous journalist and criminal who suffered fatal injuries in a car accident. His brain was transferred into a robotic body by Professor Emmett Vale, initially powered by uranium but later upgraded with a kryptonite heart after Corben discovered the mineral's capacity for unlimited energy. This transformation made him a lethal threat to Superman, as the kryptonite radiation weakened the hero while granting Metallo superhuman strength, durability, and the ability to project radioactive emissions.

In the post-Crisis on Infinite Earths continuity, beginning with reintroductions in the late 1980s such as Superman vol. 2 #1 (1987) by John Byrne, Metallo was reestablished as John Corben, a mercenary and con man whose body was reconstructed into a cyborg form following a severe accident or injury. Equipped with a synthetic kryptonite power source in his chest, this version emphasized his tragic dehumanization and recurring deaths and resurrections, often orchestrated by allies like Lex Luthor. A variant appeared in the 1970s with Roger Corben, John's brother, a soldier exposed to toxins and similarly converted into a second Metallo, highlighting familial ties and military experimentation themes.

During the New 52 era and into Rebirth, John Corben's origin shifted to that of a U.S. Army soldier critically wounded in action, who underwent experimental cybernetic enhancements incorporating kryptonite under programs linked to figures such as General Sam Lane. These developments portrayed Metallo as a product of government overreach, with enhanced capabilities including energy blasts, shape-shifting metallic components, and increased resilience. His ongoing narrative explores loss of humanity and relentless opposition to Superman, cementing Metallo as a persistent cyborg antagonist defined by his kryptonite dependency and evolving technological upgrades.