Christian replaces Maurice Joseph, who was fired earlier this month after three seasons at the helm.
Christian, 36, is a Virginia native who spent this past season at Siena after spending six at his alma mater, Mount St. Mary's. He went to two NCAA tournaments with the Mountaineers, then finished in a five-way tie for second place in the MAAC at Siena this season.
Siena went 17-16 under Christian this season with a game built on pressure defense and 3-point shooting.
Siena now has a head-coaching vacancy for the second year in a row, as the Saints fired Jimmy Patsos following an 8-24 finish in 2017-18.
Christian's brother, Jarell, coaches the Washington Wizards' G-League affiliate.
George Washington went 9-24 this season. The Colonials were 44-57 under Joseph, who replaced Mike Lonergan when he was fired after an internal investigation into players' allegations about Lonergan's behavior.
Information from The Associated Press was used in this report.