Peter O'Malley plans to leave the post on April 13, two days before Smith is due to deliver his 2011 budget proposal to the county council and detail how he will close a $144 million gap in this year's budget.
Smith, who has eight months left before he reaches his two-term limit, does not plan to replace O'Malley, a spokesman said. O'Malley's duties are to be divided among senior staff.
"Peter will be sorely missed and is not replaceable," spokesman Don Mohler said. "He is a talented administrator with outstanding organizational skills. He will be successful in all that he does and valued wherever he goes."
O'Malley, the longest-serving chief of staff during Smith's two terms, advised the county executive on political issues, set the schedule and served as the administration's point of contact with other elected leaders in the area, Mohler said.
"Given his focus on data-driven government and his work with constituencies across the county, Peter O'Malley has been an invaluable asset to my administration," Smith said in statement.
Mohler said the resignation did not come as a complete surprise.
"It is not unusual as an administration winds down that key people begin to look for other job opportunities," he said.