Moreno served 18 years on the county board representing District 4, having been first elected in 2002.
County Board Chairman Richard Brunk will appoint someone to complete the remaining two years of the term, pending full board approval.
"His resignation from the board is a huge loss for the county," Brunk said. "Pat is one of the most straight-shooting and honest individuals I've ever had the opportunity to meet. I truly believe he was doing what was in the best interest of the people he represented in his district."
Brunk said Moreno also served as chairman of the litigation committee for the past few years.
Brunk said the county board is not involved in the hiring or firing of members on the 708 board, which is an independent body. However, Moreno was not allowed to serve on both the county board and mental health board at the same time.