Origin

Gary Mitchell was born in Van Nuys, California.  He later became an outstanding Starfleet officer.  He graduated at Starfleet Academy  with top honors and was even awarded the Zefram Cochrane Psychics award.  It was at Starfleet Academy that he met James Kirk one of his professors and known for being a stack of books with legs.  Mitchell knew that in Kirk's class you either think or sink so he set Kirk up with a girl kirk almost married in a successful attempt to distract him. Not knowing this Kirk and Mitchell became friends and Kirk even requested him when he got his command on the U.S.S. Enterprise. 
 
On a mission to cross the outer barrier Mitchell was hit by an unknown force that increased his ESP powers making him a near god.  As Mitchell became more powerful he became more mad.  He became so mad that Kirk felt he had no choice but to abandon Mitchell on another planet.  There Mitchell killed his old friend Lee Kelso and escaped.  A fellow officer Elizabeth Dehner also developed these same powers but it took her a little longer and she followed Mitchell.  This left Kirk with the feeling he needed to stop Mitchell.
 
Armed with nothing more than a Type III Phaser Kirk hunted down two humans who had such powers they could sense his very thoughts and crush him with them as well.  Not feeling Kirk could do anything Mitchell let him get close but when Kirk got there he convinces Dehner that Mitchell is dangerous and she attacks Mitchell and weakens him but mortally wounds herself in the process.  Kirk eventually kills Mitchell just after his weekend state wears off by crushing him in an avalanche. 

Creation

Gary Mitchell was created for the second Star Trek Pilot from the original television series.