He has held the job since July 2006.
"We appreciate the energy and insight that Mr. Auld contributed during his tenure with Durham County," County Manager Mike Ruffin said. "We wish him well as he moves forward with other pursuits that he wants to follow."
The resignation came one day after the library's renovated Southwest Branch reopened. A new South Durham Branch is due to open later this year.
Auld, in his resignation statement, thanked the staff, trustees and Friends of the Library for their support. and said he appreciated having been "a part of the tremendous renaissance and rebirth of Durham City and County and downtown Durham."
A search for his replacement will begin immediately. Assistant Library Director Priscilla A. Lewis will serve as Interim Library Director during the search process.

The Durham Public Library's downtown headquarters is going to be in the movies.