Genosha was created by Magneto and Doctor Doom as a lush and pristine island reserved as a safe haven for mutants to escape from persecution. Its existence was kept secret in which Magneto used his electromagnetism to blind satellites to its existence, and Doom used his sorcery to keep it magically cloaked to the naked eye.
Shortly after mutants were exposed to the public, many hidden mutants fled to Genosha. Magneto and Doom manipulated the Friends of Humanity into attacking Genosha by sending an arsenal of Sentinels as part of their plan in deliberately having Superman saving the mutant island haven and making him seen as a hero to the island's residences. Superman along with Hawkgirl and Green Lantern saved Genosha, as according to the plotters' plan, but only for Superman to speak out to the enraptured mutants to only follow what is right. Soon afterward, Brainiac Mark VI crash landed offshore of Genosha and attacked the heroes.
Within several weeks after the newly-formed Justice League saved the world from the Sentinel Incident, Genosha officially opened its borders to non-mutants and subsequently formed itself as a legitimate sovereign nation, which had petitioned the United Nations for recognition in the General Assembly. The country's recognition was strongly opposed by the F.O.H. The terrorist organization abducted Genoshan Ambassador Gregory Michaels and threatened his life on public television to drop the recognition, but only to be thwarted by the Justice League.
In Marvel Comics, Genosha was located off the east coast of Africa, to the north of Madagascar, and boasted a high standard of living, an excellent economy, and freedom from the political and racial turmoil that characterized neighboring nations. However, Genosha's prosperity was built upon the enslavement of its mutant population.
The mutant oppressive regime was overthrown by the X-Men after it was discovered to allied with former X-Factor ally turned mutant hater, Cameron Hodge. A new regime was succeeded that promised better treatment of mutants, but only to see a period of general turmoil and a number of attacks by superhumans, including Magneto's Acolytes who were unwilling to forgive the former Genoshan government for its crimes against mutants. The United Nations ceded the island nation to Magneto, after he demanded an entire mutants-only nation. For a time it served as a refuge for those suffering from the Legacy Virus, but Magneto attempted to turn the mutants present into an army after Colossus sacrificed himself to disperse the cure, only to be thwarted by an emergency team of X-Men. Genosha was later destroyed by a Sentinel attack, decimating the island and killing a sizeable portion of the mutant population. In Excalibur (Volume 3), Professor Xavier worked alongside Magneto to rebuild Genosha, but goes badly as foreign military forces have thrown up a cordon around the island, no one gets in and those trying to leave are fired upon.
In the aftermath of the House of M storyline that resulted in Earth’s mutant population losing their powers or dying in the process, Magneto was among the depowered people, remaining trapped on Genosha, which had now been reduced to ruins. By the time of the Necrosha storyline, Selene resurrected the massacred residents of Genosha into her army until her defeat, and returning Genosha to an empty land.