Mark Rein-Hagen is a role-playing, card, video and board game designer best known as the creator of Vampire: The Masquerade and its associated World of Darkness games. Along with Jonathan Tweet, he is also one of the original two designers of Ars Magica. His work on World of Darkness has influenced the movie series Underworld, True Blood, and otherkin sub-cultures such as Real Life Vampires and Real Life Werewolves.
In 1987, Rein-Hagen and Jonathan Tweet founded game publisher Lion Rampant while students at Saint Olaf College; here they met Lisa Stevens who later joined the company.:232 Rein-Hagen and Tweet designed Ars Magica over a period of nine months, publishing it in 1987.:232–233 Lion Rampant encountered financial difficulties in 1990, but after Stevens pitched the idea of a merger to Rein-Hagen and Stewart Wieck,:235 they decided to merge White Wolf and Lion Rampant forming a new company White Wolf Game Studio, with the two as co-owners.:215–216Of his experience at Lion Rampant, Rein-Hagen recalls: "My father told me when I started my first game company, Lion Rampant: 'Mark, this company is going to fail, you are too young, inexperienced and poor to make it work. But, you are going to learn a lot, and next time you might just get it right.' At the time I didn't believe him, I thought we could make it, but he was right, and because of his words, I never, ever gave up."
While Rein-Hagen was on the road with Wieck and Stevens to GenCon 23 in 1990, he conceived of the game Vampire: The Masquerade which became his main project for the next year, and was published by the new company in 1991.:216 Mage (1993) was based to a certain extent on a game that Rein-Hagen had imagined back in 1989 as something like a modern-day Ars Magica, although this was the first World of Darkness game in which he was not explicitly involved.:218 Wraith (1994) marked his return to the design of the core games in the World of Darkness setting.:218 Rein-Hagen was developing a science-fiction game called Exile to be published in 1997, which was to be owned by a non-profit called the Null Foundation. However, White Wolf encountered financial difficulties in 1995–1996, which caused a falling out between Rein-Hagen and Wieck and his brother Steve Wieck. As a result, Rein-Hagen left White Wolf taking Exile with him.:222 His Null Foundation put out a playtest draft of Exile in 1997, but the game was never fully published.:222 He founded the company Atomaton, Inc. a few years later, which produced his game Z-G in 2001; Atomaton ceased operaation in 2003.:222
Rein-Hagen published Whimsy Cards, Ars Magica, and major Ars Magica supplements through Lion Rampant with Jonathan Tweet. Tweet and Rein-Hagen worked with Stevens, John Nephew, and others who would become hobby game professionals.
Rein-Hagen, along with Ray Winninger and Stewart Wieck, made major contributions to D.O.A., designed by Greg Gorden of Mayfair Games in conjunction with White Wolf, but the game was never published. It was based on a concept called "Inferno" that Rein-Hagen had worked on previously for many years at Lion Rampant, wherein players took on the roles of dead characters from old campaigns.:170
Rein-Hagen sold his shares in White Wolf in 2007 and left the gaming field.:222 He served as a writer and producer for Kindred: The Embraced, a TV show loosly based on Vampire, produced by Aaron Spelling and shown on Fox TV. He was unhappy with the finished product because FOX’s producers had a vision for the series he did not share. “The show wasn’t as good as it could have been, if they only had listened to me more.” Kindred was cancelled after eight episodes, however, following the death of its star Mark Frankel any attempts to revive it were abandoned. Rein-Hagen continued to work in Hollywood for four years total, but disillusioned. and fed up trying to make it as a writer, he decided to leave it behind. “It was the goal of my life, but finally I just left”. 
As of mid-2008 he was living in Tbilisi, Georgia, with his wife and child during the 2008 South Ossetia War. Rein-Hagen was evacuated with other US citizens living in Georgia and started the now defunct site sosgeorgia.org to help track what was happening there. He returned to Georgia after the war.
In 2012 Rein-Hagen worked on a card game called Democracy for his company Make Believe Games. This game was successfully funded by Kickstarter in November 2012. As of December 3, 2014, over two years after funding, fulfillment is largely complete. On February 4, 2014 Rein-Hagen released a statement citing poor health as the reason for his lack of communication and promising that backers would get their game. Commentators were extremely unhappy with the tone of the message and complained that Rein-Hagen's ill health had not affected his ability to work on other crowd-funded projects.
In an interview conducted in the same month for The Gentleman's Guide to Gaming, Rein-Hagen spoke fondly of his former work on role-playing games and how he is working on a new role-playing game. Rein-Hagen elaborated on this role-playing game in March 2013, in an interview on Your Main Man Speaks, describing some of the mechanics and speculating on a release date without naming it. In addition he discussed his new game Succubus: The Reborn.
- Shannon Appelcline (2011). Designers & Dragons. Mongoose Publishing. ISBN 978-1-907702-58-7.
- Larsen, Joseph (4 June 2015). "Meet Mark Rein-Hagen, Tbilisi’s Resident Game Master". Georgia Today. Retrieved 25 November 2015.
- "Mark Rein-Hagen". reddit.com. Retrieved 25 November 2015.
- "Whimsey cards". John H. Kim. Retrieved 25 November 2015.
- Appelcline, Shannon. "History of Game, #10". 3 January 2007. Retrieved 14 June 2013.
- "Let’s Remake Kindred: The Embraced!". Vampires.com. Retrieved 25 November 2015.
- "Kiindred: The Embraced, the Show We Lost too Soon". SecondShifters.com. Retrieved 25 November 2015.
- Kickstarter campign location
- "Its All in the Game!". Make-Believe Games. Retrieved 2013-02-21.
- "Democracy: Majority Rules - A Game of Politics & Negotiation by Mark Rein-Hagen — Kickstarter". Kickstarter.com. 2012-09-27. Retrieved 2013-02-21.
- "Message received on kickstarter today". Make-Believe Games. Retrieved 2014-03-13.
- on YouTube
- on YouTube
- "New Zombie RPG from World of Darkness’ Mark Rein-Hagen". News Pro. Retrieved 25 November 2015.
- "I Am Zombie: Field Manual". DriveThruRPG.com. Retrieved 25 November 2015.