He later worked at the Union Theological Seminary in New York City, where he stayed until his retirement in 1996 as John D. Rockefeller Jr.

[citation needed] Koyama died at a hospital in Springfield, Massachusetts, of pneumonia complicated by oesophagal cancer, on March 25, 2009.

Koyama was an editor of the South East Asia Journal of Theology, for which he himself wrote a considerable number of articles.

Koyama published at least thirteen books, including "On Christian Life" (currently available only in Japanese) and over one hundred scholarly articles.

Koyama's work has been described as helping to bridge the boundaries between East and West, between Christianity and Buddhist thought, between the rich and the poor.

He also explained in the preface how the book grew out of the Earl Lectures which he delivered at the Pacific School of Religion in California.

He also told, in this book, of an incident which happened to him in Singapore, where he met a Buddhist monk from Sri Lanka.