When Harvard's first purpose-built library building, Gore Hall, was opened in 1841, he was appointed the assistant librarian under Thaddeus William Harris.

On May 30, 1866, Sibley married Charlotte Augusta Langdon Cook, at Sommerville, Massachusetts.

[2] Sibley oversaw both the physical and fiscal expansion of the Harvard Library.

He also compiled the initial volumes of Sibley's Harvard Graduates series (three volumes published between 1873 and 1885 covering the classes of 1642 to 1689) and bequeathed funds to the Massachusetts Historical Society for continuation of this project.

[note 1] Sibley died at his home in Cambridge on December 9, 1885.