Marcus was known as a pastor, teacher, spiritual advisor, and conflict mediator. He began his career as a pastor at Portland Mennonite Church, where he spent sixteen years and developed lifelong connections. He was an associate professor at the Associated Mennonite Biblical Seminary from 1982–1999, and an adjunct professor at Eastern Mennonite Seminary from 1999 to the time of his death.
Marcus was a longtime member of Spiritual Directors International and a servant leader on the Presence journal editorial review panel. He specialized in conflict mediation, bringing a healing ministry to hundreds of pastors and churches struggling with divisive issues. He was an expert teacher in spiritual formation, mentoring individuals, and teaching at the seminary level.