The 'Disposable Teens' artist used Manson's surname as part of his stage name, along with Marilyn Monroe. The practice was also used by other members of his band, including Twiggy Ramirez, Ginger Fish and Madonna Wayne Gacy.
The postcard from Manson was leaked on Loudwire, and contains strange, threatening remarks made to the singer.
Charles Manson wrote: "To Marilyn Manson, It's taken me a long time to get there from where I could touch M. Manson. Now I got a card to play - you may look into my non-profit, ATWA, and give Manson what you think he's got coming for Air, Trees, Water, and you (sic).
"Or I will pay Manson what you think Manson got coming - the music has make Manson into Abraxas Devil, and I'm SURE you would want some of what I got from what I got. It's a far out balance."
The letter continued: "Beyond good and bad, right, wrong. What you don't do is what I will do - what you did a sing-along, and let it roll and said how you saved me a lot of steps - I don't need, it's not a need or a want. Couped - coup. Ghost dancers slay together and you're just in my grave Sunstroker Corona-coronas-coronae - you seen me from under with it all standing on me. That's 2 dump trucks - doing the same as CMF 000007. Charles Manson."
Charles Manson was behind a series of murders in the US in 1969, including that of pregnant actress Sharon Tate, who was married to director Roman Polanski at the time. He is currently in California State Prison in Corcoran.
Marilyn Manson will join Rob Zombie for a joint UK arena tour later this year, including a gig at London's O2 Arena on November 26.
Watch Marilyn Manson talk to Digital Spy below: