Lazareth earned a bachelor's degree in history in 1948 from Princeton University, Princeton, N.J. In 1953 he earned a master of divinity degree from the Lutheran Theological Seminary at Philadelphia (LTSP). He earned a doctorate in doctrinal theology from Columbia University — Union Theological Seminary, New York, in 1958. He was also the recipient of seven honorary doctorates. He was a faculty member of LTSP for nearly 20 years. In 1976 he became director of the Department for Church in Society, LCA Division for Mission in North America. He served three years as director of the WCC Faith and Order Secretariat, then became pastor of Holy Trinity Lutheran Church, New York, where he served from 1983 to 1987. He was visiting professor, Union Theological Seminary, New York, from 1987 to 1996. From 1991 to 2002, he was co-president of the Lutheran-Eastern Orthodox International Doctrinal Dialogues, Lutheran World Federation, Geneva.
In 1988 Lazareth was elected bishop of the ELCA Metropolitan New York Synod and served in that role until 1992 when he retired from active ministry. From 1996 to 2003, he was a member of the executive staff, Center of Theological Inquiry, Princeton, and was a visiting professor at Princeton University before he joined the faculty at Carthage College in 2003. He authored 13 books, edited 15 books and wrote at least 45 essays. In 1995 he was named "Lutheran Pastor of the Year USA" by the Luther Institute, Washington, D.C.
He died 2008.