They offered Crestwood Publications' office manager Nevin Fidler, who knew the mechanics of distributors and other necessary vendors, a piece of the company to manage the operations. Simon and Kirby invested their own savings in the new company, working with veteran paper and printing broker George Dougherty, Jr. To distribute the comics, they chose Leader News, the company that distributed Bill Gaines' EC comics. Their intent was to create comics for the adults of the 1950s who had read comics as children in the 1940s.
However, the closure of many comics publishers soon meant that distributor Leader News no longer had enough money to advance to small companies like Mainline, and when that company closed it brought about the end of Mainline. The unpublished materials for Mainline's titles were sold to Charlton, which published them and continued some titles under new names. With the demise of Mainline, the longstanding partnership between Simon and Kirby also ended, although they would collaborate on a few more comics. Simon left comics for a time to take up a role in advertising. With Simon's blessing, Kirby took Challengers of the Unknown, the final project of the Simon & Kirby team, to DC.
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