Donald H. Blevins, Los Angeles County’s chief probation officer, announced his resignation Thursday to the Board of Supervisors in a letter saying he was stepping down for “personal reasons,” a Probation Department spokeswoman said.

Blevins, 59, took over the department in April 2010 at a time when it was under orders from the U.S. Justice Department to improve juvenile detention facilities.

He leaves at an equally challenging time when the state is shifting nearly 4,000 parolees to county Probation Department supervision before the end of the year.

Blevins’ resignation is effective at the end of next week. He had alerted his 6,200 employees of his decision in a department-wide e-mail message last Friday.

“My departure in no way diminishes the pride and respect that I have for the thousands of employees who make up the Los Angeles County Probation Department,” Blevins’ e-mail said.

“It has been an honor to roll up my sleeves with you to work on servicing the critical needs of our clients and our community,” it said. “This department has hardworking and dedicated staff that makes a difference in the lives of those we serve every day.”

The No. 2 person in the department, Deputy Probation Officer Calvin Remington, will serve as chief probation officer until further notice, spokeswoman Kerri Webb said in a statement.