[1] Heinsoo was the lead designer on the 4th Edition of Dungeons & Dragons (2008), and is co-designer of the 13th Age roleplaying game along with Jonathan Tweet.

[citation needed] Chaosium hired Heinsoo in 1996 to oversee their licenses for Glorantha,[3]: 359  but he was laid off the following year.

[3]: 361 Heinsoo joined Wizards of the Coast (WotC) as part of the "D&D Worlds" team, where he focused on the third-edition version of Forgotten Realms.

He also helped write the third edition Forgotten Realms Campaign Setting, which reached the top 50 of the non-fiction bestsellers in Canada in 2002[4] and won an Origins Award for best roleplaying supplement of 2001.

[9] His book Monster Manual 2, co-written with Chris Sims, was a Wall Street Journal bestseller in 2009.