Sean Reynolds was born in Chula Vista, California, and grew up there. He was introduced to the Dungeons & Dragons game (with the red Basic Set and the blue Expert Set) in 1980 by his cousin, and soon began playng the AD&D game.

Reynolds began working for Time Warner Interactive in 1994, to develop their America Online and websites. Reynolds first entered the role-playing game industry in 1995, when he was hired by TSR as their online coordinator, beating out Bruce Cordell for the same position. According to Reynolds, "I had two goals: to get TSR a website, and to change the company's then-restrictive online policy to something more reasonable." When TSR was bought out by Wizards of the Coast, Reynolds moved to Washington and worked on Wizards' pre-existing website. Reynolds considers one of his greatest achievements of this time to be the fact that he got TSR/Wotc to relax its "draconian" online policy--enforcement of which in the past was the cause of much bad blood between the company and fans. Wizards of the Coast had a much more open online policy, so Reynolds applied for a job in the company's R&D division as a game designer. He designed such products as The Scarlet Brotherhood for AD&D, Beyond Science: A Guide to FX for the Alternity game, and The Green Goblin's Guide to Crime for the Marvel Super Heroes Adventure Game. Reynolds also did some design work such as writing RPGA adventures and contributing to Children of the Night: Ghosts.

By 1998, Reynolds had left his web position and was working full time for WotC as a designer as part of "Team Greyhawk," the creative team tasked with revitalizing TSR/WotC's oldest campaign setting.

Reynolds also contributed to the 3rd edition D&D Monster Manual, introducing some new creatures such as the krenshar. Reynolds was laid off from WotC in 2002, and since has formed his own self-publishing company, "Sean K Reynolds Games." He continues to freelance in the d20 industry while working as an IP developer at Upper Deck.

Books he has worked on for the d20-based 3rd edition include the Living Greyhawk Gazetteer, Forgotten Realms Campaign Setting, Savage Species, and Ghostwalk (co-written with Monte Cook).

Reynolds has been a lacto-ovo vegetarian since 1992. He likes animals and has four cats. He plays some sort of roleplaying game typically once per week.