Congregational churchman and writer who founded and led Christian Endeavor.

Clark graduated from Dartmouth College in 1873 and Andover Theological Seminary in 1876. He was pastor of churches in Portland, Maine (1876–83), and South Boston (1883–87). In 1881 he founded the United Society of Christian Endeavor (International Society of Christian Endeavor), an autonomous interdenominational organization of young people. Its members pledged to lead a Christian life, to pray to God and read the Bible every day, and to participate actively in the work of the church. In 1887 he resigned his pastorate to serve as the organization's president.

Clark became president of the World Christian Endeavor Union in 1895 and traveled around the world five times in the interests of its work. Clark also edited The Christian Endeavor World (1887–1919) and wrote many books on religious themes for young people.