Little is known of the company despite visual clues at their sites, and desciptions from the level select screens of both games.
Their sites are frequent (yet unwilling) hosts for Carmageddon events, and sometimes even the management get involved: One racing locale is the "MagnaChem Rural Development", and within the grounds is the jail where many of the M.C. Board of Directors reside.
When MagnaChem screw up, they don't do it by halves. They "accidentally" (no-one can prove otherwise, especially with their laywers) turned the Pacific Ocean 99% Sulphuric in mid 2000, although it must be noted that in most environments the water is normal. The "MagnaChem Acid Ocean Reprocessor" facility was created in an effort to resolve this, but what have they done about the region designated "MagnaChem Environmental Disaster"? Once all forest and lakes, it is now a poisoned network of surreal land, caustic acid waters, and bizarre twisted ruins. The place is said to be the fevered vision of a complete lunatic; The C.E.O. of MagnaChem, who was personally in charge of the project.
At the remote "MagnaChem Abandoned Ice Station", workers unearthed the frozen remains of warriors from a long defunct clan. As many people would suspect of a wealthy corporation, they also seem to have made dealings with the Devil: MagnaChem has a storage facility in Hell.
The Cops show up in M.C.'s holdings, which presumably means they're subject to federal law enforcement - or they can afford to hire police privately.
In Carmageddon: Reincarnation the company is called MagNuChem. They look to now have different names for different arms of the corporation, as MagNuAgri Corp is managing/spreading bovine plague in Dusty Trails.
Despite being barely mentioned in the games, it is worth pointing out that this company is fictional, and bears no relation to Magnachem International Laboratories Inc. (an anti-cancer development company based in Boca Raton, Florida), or any of the several other pharmaceutical companies with the same name.