He said more than 16,000 have been adopted from foster care since he became director.
The agency has been criticized repeatedly for years after children in its care were killed even though DHS workers had been warned about abuse.
The criticism intensified last year after a 5-year-old girl, Serenity Deal, was murdered by her father in Oklahoma City.
Serenity died in June from a severe head injury less than a month after she began living with her father at the recommendation of DHS workers. Serenity was placed with her dad even though she was injured twice in January 2011 during overnight visits with him.
DHS subsequently suspended four workers involved with the case. Two were later fired, one committed suicide and the fourth quit.
In his letter to employees Tuesday, Hendrick wrote, “While some have been critical of some performance in child welfare, the prevention efforts our agency and others have taken have been extremely successful.”
The agency also faced criticism last year because of disclosures in the federal class-action lawsuit that DHS commissioners had exercised little oversight of decisions by Hendrick and other DHS officials.
In an editorial Jan. 15, The Oklahoman called for Hendrick to step aside because the agency “needs new energy, enthusiasm and ideas.”
In a brief statement, Gov. Mary Fallin thanked Hendrick for his many years of public service and wished him the best.
House Speaker Kris Steele praised Hendrick for his leadership in improving the adoption process in Oklahoma. Steele, R-Shawnee, said Hendrick also has done a lot to update technology and to improve child support collections and food stamp distribution.
Steele acknowledged meeting with Hendrick last week to discuss the director's future.
“I had a discussion with Director Hendrick, and maybe other legislators did, as well,” Steele said. “We talked about his current situation, and I affirmed his contributions to the agency. ... He is a fine man. But we also talked about the potential need for a fresh start.”
Steele pointed to a lack of cooperation with the Legislature as one of Hendrick's weaknesses. The speaker said he hopes that is an area that will improve once a new director is selected.
The DHS commission's chairman, Brad Yarbrough, praised Hendrick for his faith in God, compassion, intellect and service.
Yarbrough also praised the DHS workforce, saying he was confident workers would meet and solve the challenges of the future. “It will be a team effort,” he said.
Hendrick said he plans to read books, spend time with his grandchildren and take some time to contemplate what to do next before taking on another career challenge.
Hendrick stayed much longer in his position than officials in similar jobs in other states.
In his letter to employees, Hendrick wrote, “The average tenure of a state human services director nationally is about two to four years. Occasionally, some directors stay for seven to 10 years. The main motivation for my staying for 13-and-a-half years is the inspiring and creative work I have seen so many of you do for so many vulnerable families.”