BALTIMORE — A memorabilia collector and self-styled expert on presidential history pleaded guilty Tuesday to conspiring to steal thousands of documents signed by leaders throughout U.S. history.

Barry Landau, whose knowledge of the White House earned him appearances on network morning shows, admitted in the plea to taking documents from the Maryland Historical Society and conspiring with his assistant to steal documents from several institutions with the intention of selling them.

Thousands of documents were seized by authorities over the summer from the 63-year-old Landau’s artifact-lined Manhattan apartment. Prosecutors say he schemed for years, if not decades, to steal valuable documents.

The assistant pleaded guilty in October to the same charges: theft of major artwork and conspiracy to commit theft of major artwork....