Professor Klein was educated in Art History, Early Christian Archaeology, and German Literature at the universities of Freiburg im Breisgau, Munich, London, and Bonn. His research focuses on Late Antique, Early Medieval, and Byzantine art and architecture, the cult of relics, and issues of cultural and artistic exchange.
From 2004–2007, he served as the Robert P. Bergman Curator of Medieval Art at the Cleveland Museum of Art and continued to oversee the reinstallation of the museum's renowned collection of Medieval and Byzantine art until 2010. His work as a curator includes various national and international loan exhibitions, among them Restoring Byzantium. The Kariye Camii in Istanbul and the Byzantine Institute Restoration (Wallach Art Gallery, 2004), Medieval Treasures from The Cleveland Museum of Art (Bayerisches Nationalmuseum/The J. Paul Getty Museum, 2007–08) and Treasures of Heaven. Saints, Relics and Devotion in Medieval Europe (Cleveland Museum of Art/Walters Art Museum/British Museum, 2010–11).
In 2011, he received the 50th annual Mark Van Doren Award for Teaching, which honors a Columbia professor for commitment to undergraduate instruction as well as for "humanity, devotion to truth, and inspiring leadership."