Noted founder and C.E.O. of BEE TRAIN animation studios.

In the mid 1970's he began his animation carrer as a director with Tatsunoko Productions, debuting as an Assistant Director on Time Bokan (1975).

After surviving a severe skiing accident in the mid 80's Mashimo was hospitalized for a lengthy duration, where he conceived his idea as a "hospital studio" to nurture the creative spirit of animators over a commercial focus. In June of 1997 he set up BEE TRAIN under Production I.G. In 2006 his studio became independent.

Koichi Mashimo was also apart of Project .hack and the .hack Conglomerate, which included some notable names in the industry such as Yoshiyuki Sadamoto of GAINAX, Diasuke Uchiyama of Bandai, Kouji Morimoto of Bandai, Shinichirou Inoue of Kadokawa Shoten, Fukashi Azuma of TV Tokyo, and Kazunori Ito of Catfish.

Other non-anime staff include a novelized version of Eat-Man and Production for Xenogears animation cut scene footage as well as Wild Arms 3 for the PS2.