Magog first appeared as an antagonist in the series Kingdom Come. He killed the Joker causing the public to praise him as a hero. This caused Superman (who disliked Magog's methods) to withdraw from the world. During Superman's exile, superhumans become more violent like Magog. At an unknown time, Magog formed the Justice Battalion. They battled Parasite, which led to him literately tear Captain Atom apart, causing him to explode like a A-Bomb, with Alloy and Magog being the only survivors. This lead Superman to return from his exile and reform the Justice League. They found that Magog was sorry for what he has done and was traumatized. He was sent to a superhuman prison and spent most of his time in his cell. When the rogue superhumans were freed and battled the JLA, Magog avoid fighting and tried to save as many lives as he can. At the end of the series, he has decided to retire to Paradise Island. Note these events take place on Earth-22 and are non-cannon.
Justice Society of America
Introduced into the main continuity, this version of Magog was once Lance Corporal David Reid and fought in the War of Iraq. He touched a "stone fragment of the Old God Gog", which filled his body with plasma energy, but ruin his left arm. He would later gain a lance-shaped device that allowed him to control it. He was allowed to join the JSA, because he is the great-grandson of Frank D. Roosevelt. They helped a person known as Gog fight against a rogue military attack. Gog turned Magog's right eye and ruined armed to gold. It turns out that Gog was actually evil and the Society turns on him with Magog killing him. Magog left to join his family farm. He rejoined the team, but clashed with the other members due to his military personality.
He quit the JSA again and joined a team called the All-Stars, which was composed of mostly younger heroes.
Magog got his own series. In the first story arch, he battled against the Flashpoint, a mysterious weapon making group run by D. P.Macklin. This series also fleshed out the character some more in various ways such as his origin and new rogue gallery. Macklin framed Magog and (even though he cleared his name) he got kicked out of the All-Stars. Magog discovered the Cult of Gog was manipulating him and he seemly died stopping them. But, it later seen alive.
Magog tried (and failed) to capture Captain Atom, who was framed by a telepathic Maxwell Lord. Maxwell Lord met a being known as the Entity, who revealed that Magog could cause a war among Earth's superhumans (akin to Kingdom Come). After Magog tracked down Captain Atom, Atom manged to convince Magog that Lord was manipulating him. Lord (unhappy about this) killed Magog, but used his powers to make it seem like that Atom did it.
PowersMagog's body is full of plasma energy that he controls via his staff. He has super strength and durability (he was seen surviving a nuke). He can fly. He was once seen being able to teleport people, but this power has not been seen after he killed Gog. He is able to use various forms of super-vision in his "blind" eye. According to Keith Griffen (who wrote his solo series), he actually has more power than he knows about.