Koichi Motohashi established the anime studio Nippon Animation Co., Ltd. in June 1975. The studio quickly found success with such series as Maya the Bee and Dog of Flanders. Motohashi served as the president of Nippon Animation from June 1975 to October 2010. In addition to serving as the president of Nippon Animation, he served as the executive producer for the acclaimed World Masterpiece Theater collection. Hayao Miyazaki and Isao Takahata, who would go on to form Studio Ghibli, directed several episodes of the World Masterpiece Theater series.

Motohashi served as an executive producer for both film and television. The films he served as the executive producer on were The Legend of Manxmouse from 1979, Locke the Superman from 1984, Mirai Shounen Conan Tokubetsu Hen-Kyodaiki Gigant no Fukkatsu from 1984, and Tottoi from 1992.

Motohashi served as an executive producer for television series from 1975 through 2010. In the 1970s, he served as the executive producer for Dog of Flanders, Laura, a Little Girl on the Prairie, Sindbad the Sailor, From the Apennines to the Andes, Attack on Tomorrow, Ginguiser, Monarch: The Big Bear of Tallac, Rascal the Raccoon, and Anne of Green Gables. In the 1980s, he served as the executive producer for Adventures of Tom Sawyer, Alice in Wonderland, A Little Princess Sara, Seishun Anime Zenshu, Ai no Wakakusa Monogatari (Little Women), Little Lord Fauntleroy, New Grimm Masterpiece Theater, Toppo Jijo, Yume Miru Toppo Jijo, Jungle Book: Shounen Mowgli, and Peter Pan no Boken.

In the 1990s, Motohashi served as the executive producer for Chibi Maruko-chan, Moero! Top Striker, Trapp Ikka Monogatari, The Bush Baby, Christopher Columbus, Wakakusa Monogatari Nan to Jou Sensei (Little Men), Ai to Yuuki no Pig Girl Tonde Buurin, Tico and Friends, and Chibi Maruko-chan TV 2. In the 2000s, he served as the executive producer for Legend of the Condor Hero, Les Miserables – Shoujo Cosette, Before Green Gables, and Soreike! Zukkoke Sanningumi.

Motohashi served as a producer on television series in the 1970s and the 1980s. In the 1970s, he produced Angie Girl and Song of Baseball Enthusiasts. In the 1980s, he produced Swiss Family Robinson and Ai Shoujo Pollyanna Monogatari. In 1987, Motohashi also served as the executive animation producer for Grimm Masterpiece Theater.

Anime News Network reported on Thursday, October 28, 2010 that the anime studio Fanworks announced on their Twitter account that Koichi Motohashi passed away. He was 80 years old at the time of his death.