Koster is widely recognized for his work as the lead designer of Ultima Online[1] and the creative director behind Star Wars Galaxies.

Koster attended Washington College in Chestertown, Maryland, receiving a bachelor's degree in English (creative writing) and Spanish in 1992.

The same year he became involved with MUDs as a developer of Worlds of Carnage, then in 1994 moved on to become implementor of LegendMUD, where he was known as Ptah.

Promoted to chief creative officer in 2003, he relocated to Sony Online Entertainment in San Diego[6][7] where he spent three years primarily responsible for business development.

In 2019, Koster and Eric Goldberg founded a new company, Playable Worlds, focused on producing the MMORPG, Stars Reach.