Short answers, pulled from the story.
Mike Mearls began writing adventure modules for third-party publishers in 2001. He released To Stand on Hallowed Ground and Swords Against Deception that year before crafting Fear the Worst in 2002.
Wizards of the Coast hired Mike Mearls in June 2005 to join the Flywheel design team alongside Andy Collins, David Noonan, and Jesse Decker. He assumed lead designer duties for the franchise after Rob Heinsoo left the company in 2009.
Mike Mearls served as senior manager for research and development while co-leading the creation of Fifth Edition alongside Jeremy Crawford. By 2018 he held the title of Creative Director for the entire franchise.
Mike Mearls left the Wizards of the Coast tabletop RPG team in 2019 to join the Exploratory and Vision design teams for Magic: The Gathering. Ray Winninger replaced him as Executive Producer in charge of the Dungeons & Dragons studio in 2020.
Chaosium announced Mike Mearls as their new Executive Producer of role-playing games on the 1st of May 2024. News emerged by June 2025 that he had left Chaosium while still working on projects like RuneQuest before departing to take a position at Asmodee.