Jemas was appointed as executive vice president of Marvel Entertainment Group in 1993. In January 2000, he became president of consumer products, publishing and new media — essentially Marvel Comics' publisher. During his tenure, with editor-in-chief Joe Quesada, the company experienced a rapid growth in profitability and critical acclaim. This was in part from the popularity of the Marvel Knights imprint that was created during his tenure. However his tendencies toward micromanagement and conscious stirring-up of the fan community made him a controversial figure.
During his tenure at Marvel, he created the Ultimate Marvel Universe. Other later initiatives, such as the titles Trouble, Marville and The Call were less successful. Upon his departure from the company, Jemas continued to work with Marvel in a non-executive capacity through mid-2004. He also wrote or co-wrote Marvel comic books including Namor, Origin, and Marville.
Comic-book writer Gail Simone said on her blog that she ghost wrote for Jemas during his tenure at Marvel She later clarified that she was referring to a comedic article about Marvel giving away free cookies rather than an actual comic.
Upon his departure from Marvel Entertainment, Jemas started 360ep, an entertainment property management firm. He developed the Freeware Bible, which translates ancient Aramaic-Hebrew words into English synonyms. and a t-shirt line, IDtees