In 1907 he moved to the United States to become the first curator of the department of decorative arts in the Metropolitan Museum in New York City.
After returning to Europe to serve in the German Army in World War I, Valentiner later was appointed to other positions in the US.
During the early 1930s, he commissioned Mexican artist Diego Rivera to create a 27-panel mural series about Detroit industry for an interior court of the museum, and gained the patronage of Edsel Ford for the project.
While controversial in content, the work attracted thousands of new visitors and led to the museum being granted a larger budget by the city.
He studied at Heidelberg under Henry Thode, and in the Netherlands with Cornelis Hofstede de Groot and with Abraham Bredius.
He commissioned Mexican artist Diego Rivera to create a series of murals after having seen his work in San Francisco, California.
Valentiner developed an expert staff of curators, a vision of an encyclopedic collection, and the museum as a resource for the city, the state, and the Midwest.
[citation needed] In 1955 Valentiner was appointed as the first Director of the North Carolina Museum of Art in Raleigh, the capital.