Robert Samuel Maclay was born on February 2, 1824 in Concord, Pennsylvania, the son of Robert Maclay and Annabella Erwin Maclay, one of nine children. His parents were highly respected members of the community, running a tanning business and actively involved in the Methodist Episcopal Church. Maclay entered Dickinson College in the fall of 1841 and was elected into the Belles Lettres Society. He graduated with a Bachelor of Arts degree in 1845, received his Masters in 1848, and was later honored with a Doctor of Divinity from his alma mater
. One year after his graduation, Maclay was ordained in the Baltimore Conference of the Methodist Episcopal Church. At this time, the Church was suffering the internal struggles that the heated debate over the issue of slavery brought on. In 1847, however, Maclay was appointed as a missionary to China, where he began a lengthy missionary career overseas.
Maclay arrived in Fuchau, China on April 12, 1848. He spent 23 years in China learning the language, establishing schools and churches, and preaching the Gospel. While in China he published two books: Life Among the Chinese with Characteristic Sketches and Incidents of Missionary Operations and Prospects in China (1861) and an Alphabetic Dictionary of the Chinese Language in the Foochow Dialect that he completed with Reverend C.C. Baldwin in 1870. In 1871, Maclay returned to the United States where he was appointed superintendent of the newly-founded mission in Japan. Maclay arrived in Yokohama on June 12, 1873 and immediately set about learning the language and seeking converts. He became an integral part of the Wesleyan mission in Japan, helping to found and serve as first president of what is now the Ayoma Institute in Yokohama. While serving in Japan, Maclay was asked to travel to Korea to survey the possibility of a Methodist mission there. In June, 1884, Maclay made a brief visit to Seoul, where he acquired the permission of the king to begin medical and educational mission work. He declined leadership of the mission, though, and returned to Yokohama.
Maclay retired from the mission field in 1887 and returned to San Fernando in California. He became the dean of the Maclay School of Theology, named for his brother, Senator Charles Maclay, and spent the remainder of his life as an educator. Maclay was first married on July 10, 1850, in Hong Kong, to Caroline Henrietta Sperry of Brooklyn, New York. Caroline died suddenly in Japan in 1878. They had eight children, six born in East Asia, but only four lived past childhood. In 1882, he married Sarah Ann Barr, with whom he had no children. Robert Samuel Maclay died on August, 18, 1907 in Los Angeles, California.