He is co-developer of DikuMUD, a popular multiplayer text-based role-playing game codebase, and former chief executive officer of Sitecore, a global customer experience management software company, which he co-founded in Copenhagen, Denmark in 2001.

[3][4] From the age of 11, Seifert spent summers and Christmas holidays in San Rafael, California with his father, who immigrated to the United States and founded the microcomputer products firm Sun-Flex Company, which was later sold to Xidex Corporation.

[10][11] It was at DIKU (Danish: Datalogisk Institut, Københavns Universitet)—the department of computer science at the University of Copenhagen—where Seifert got involved in the DikuMUD project and also met the colleagues with whom he would later found Sitecore.

In June 1990 at DIKU, Seifert joined Hans Henrik Stærfeldt, Tom Madsen, and Sebastian Hammer (and later Katja Nyboe) to work on the development of DikuMUD, a multiplayer text-based role-playing game, which is a type of MUD.

The group recognized a growing demand for their website services and decided to turn Thrane's invention[14] into a marketable product (that would today be classified as a content management system).

[1] In 2013, Seifert joined the likes of Skype co-founder Janus Friis and Turbo Pascal author Anders Hejlsberg as a recipient of Denmark's annual IT Prize (IT-Prisen) for lifetime achievement in the field of information technology.