Seifert was born in Copenhagen, Denmark in February 1969 to Erik J. Thomsen and Kirsten Seifert, who divorced when he was two years old. His great-grandfather was Carl Seifert (d. 1935), a Danish blacksmith and manufacturer who was recorded in the Kraks Blue Book (Kraks Blå Bog) of well-known Danes in 1929.

Throughout his childhood and high school years, Seifert resided with his mother and two brothers on the Danish island of Bornholm. 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. There, his father co-invented an anti-glare device for computer terminals, which was awarded United States Patent number 4,253,737 in 1981. It was during the first summer holiday with his father that Seifert became interested in computers and wrote his first computer program. At age 15, with the help of a friend and his mother, Seifert started his first IT company, Danbyte, which imported computer disks to Bornholm.

Seifert and the team released the DikuMUD source code in October 1990 and it became the root of one of the largest trees of derived code from a MUD-like source code package. After the last official release of DikuMUD in July 1991, the team moved on to the development of DikuMUD II, which continues to run today under the name of Valhalla MUD.

Seifert started Pentia in 1998 as a systems integration company focused on Microsoft technologies. As consultants, Seifert and his team were hired to design and implement websites, among other things, for a number of large Danish international corporations. To make their jobs easier, Thrane had devised a set of automated tools and methods for developing and managing websites. At the time, making changes to a website required the expertise of a programmer or developer. The group recognized a growing demand for their website services and decided to turn Thrane’s invention into a marketable product (that would today be Classified as a content management system).

In 2001, Seifert and his Pentia co-founders spun off Sitecore as a separate Business entity, which initially sold content management systems in the Danish market but has grown profitably into a recognized global provider of customer experience management software with more than 850 employees and competition that includes IBM, Oracle Corporation, Salesforce.com, and Adobe Systems. Seifert has served as CEO since 2001 and was initially joined by Ole Sas Thrane and Jakob Christensen.

Sitecore expanded Business into the United States in 2004. Seifert moved to Kentfield, California, United States with his wife and three children in 2013. He works out of the Sitecore USA headquarters in Sausalito, California.

Seifert is credited as the co-inventor of a method for collecting human experience analytics data, which was awarded United States Patent number 8,255,526 in August 2012.

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.

In 2014 Seifert was included in the Kraks Blue Book (Kraks Blå Bog).