In January 1887, Dinsmore was appointed by President Cleveland as Minister Resident and consul general to the Kingdom of Korea and served until May 25, 1890. Because of his acquaintance with Korea and Korean matters, he was sought out by a young Syngman Rhee in January 1905, while he was on his diplomatic mission United States to secure aid for Korea against Japanese annexation. Dinsmore succeeded in getting Rhee a brief meeting with Secretary of State John Hay, but Rhee's mission would ultimately end in failure.