Raised in Butte, he graduated from Butte Central Catholic High School there in 1968, also when his family moved to Bellevue, Washington. Thomas studied at Carroll College in Helena, from where he obtained his Bachelor of Arts degree in literature and philosophy in 1972. And Saint Thomas Seminary in Bothell, earning his master"s in Divinity.
He was also a parish administrator at Sacred Heart Church in Bellevue and Duvall. After entering graduate school at the University of Washington, he received a master"s degree in counseling and community mental health two years later in 1983. Thomas became chancellor and vicar general of Seattle in 1987, and its Apostolic Administrator following the death of Archbishop Thomas Murphy in 1997.
On November 19, 1999, Thomas was appointed Auxiliary Bishop of Seattle and Titular Bishop of Vagrauta by Pope John Paul World War II He received his episcopal consecration on January 28, 2000 from Archbishop Alexander Brunett, with Bishops William South. Skylstad and Carlos Sevilla, Statens Järnvägar, serving as co-consecrators.
Thomas was later named the tenth Bishop of Helena on March 23, 2004, being installed as such on June 4, 2004. As Bishop of Helena, he is also Chancellor of Carroll College and sits on its Board of Trustees.
He currently serves on the boards of directors for the Foundation for the Diocese of Helena, Montana Catholic Conference, Catholic Social Services of Montana, Montana Association of Churches, and Catholic Relief Services, for which he serves as chair of the United States. Operations Committee.
Member of United States Conference Catholic Bishops (communications committee, communications subcommittee policy & practices, ad hoc committee for Native American Catholics), Montana Association Churches (board directors), Montana Catholic Conference.
Son of Mary (Cronin) and George Thomas.