A cradle Episcopalian and a Houston native, McClain is a former high school teacher with a bachelor's degree in biology and history from Trinity University in San Antonio. After experiencing a call to ordination in 1974, she earned a master's degree in religion from the Seminary of the Southwest and eventually was ordained in 1985.
She served at Resurrection, Austin, and at Epiphany, Houston, before moving to Arizona in 1989. There, she was involved in developing rural ministry and stewardship. In 1992, she was named Canon to the Ordinary and, three years later, became dean of Trinity Cathedral. In 2005 she was named executive director of the national Church Deployment Office (CDO).