From the Super Mario Wiki, the Mario encyclopedia
Stephen Mortimer is a game designer and former Nintendo employee involved chiefly in the production of puzzle games such as those in the Mario vs. Donkey Kong series. He enrolled at the Nintendo Software Technology Corporation as an intern while studying at the DigiPen Institute of Technology, his first game being Mario vs. Donkey Kong 2: March of the Minis. For the Mario vs. Donkey Kong series, he specifically designed or assisted design of the games' integrated level editors, as well as mechanics such as Pink Blocks and boss battle behaviors. He also designed a large number of levels for each game, both shipped along with it or released subsequently as online stages. Mortimer gained more prominent roles with each game in development, eventually taking over director duties for all games starting with Mario and Donkey Kong: Minis on the Move and becoming the youngest director for a title shipped by Nintendo. He left the company following the release of Mini Mario & Friends: amiibo Challenge and is currently a senior game designer at Riot Games in Los Angeles.
Mortimer is featured in a YouTube video posted on Nintendo's channel, in which he offers tips for level creation in Mario vs. Donkey Kong: Tipping Stars. He was known as "Mort" on Miiverse, where he consistently updated the Mario vs. Donkey Kong: Tipping Stars community with posts regarding new official online levels, which he also produced. After leaving Nintendo, he opened a YouTube channel, "First 30 of Game Design", where he played and discussed the game design of various video games, including Paper Mario: Color Splash and Super Mario Run, before the channel went inactive.