Outgoing Administration for Children's Services chief John Mattingly was a highly respected child welfare expert, but his seven-year tenure at the agency was often overshadowed by high-profile child deaths.

Mattingly, who was appointed in 2004, resigned Tuesday, and Mayor Michael Bloomberg announced Wednesday that family court judge Ronald Richter was selected to be his successor.

The most recent blow to Mattingly's reputation was a case involving the death of an ailing 4-year-old girl whose medical condition was overlooked by caseworkers who were not properly monitoring her family.

Her mother has been charged in the girl's death and, in an unprecedented move, the Brooklyn District Attorney also indicted the caseworkers involved and began investigating whether systemic failures lead to the child's death.

Despite Bloomberg's backing, the incident sparked calls for Mattingly's resignation.

"I think there were just too many instances of failure to follow up on cases of possible abuse," said City Councilman Oliver Koppell, who was among those calling for Mattingly to step down, "and I think new leadership has been needed for quite some time."

The 2006 beating death of 7-year-old Nixzmary Brown captivated the city and resulted in a shakeup of the child welfare system, exposing flaws in an overburdened system dogged by high turnover and an increasing number of abuse and neglect reports. 

"We have to do it right every time, every day," Mattingly said in a 2006 interview. "The public is right to expect that of us, but there will be circumstances where we make mistakes because we gathered on all the facts but we didn't have enough to act on. On the other hand, we will be there and we will continue to get stronger as we go along."

Several child advocates say deaths within this vast system are inevitable and they give Mattingly credit for trying to correct mistakes.

"I think that he dealt with every instance honestly and transparently and tried to get to the bottom of what had occurred in the agency to make sure that practice improved moving forward, and I think that’s respectable," said Jennifer March Joly, director of Citizens’ Committee for Children.

Mattingly never panicked during the high-stakes Nixzmary Brown case, according to former communications director Sharman Stein. He remained focus on reforms that included hiring former police detectives to help bolster investigations and implementing Child Stat, a process by which casework is reviewed in order to catch mistakes early, she said. 

Stein said it was unlikely Mattingly was leaving his post because of the latest blemish on his tenure: "I would say that he would have kept going if it weren't that his family needed to come first at this time," she said.

ACS is a fairly new agency. It came into existence in 1996 and has gone through three commissioners. At seven years, Mattingly served the longest.

According to some experts, the position of child services chief is a high turnover job with an average length of stay of just over two years. 

"Many political leaders will fire their directors when there’s a child death," said Shay Bilchik who conducts child welfare research at Georgetown University’s Public Policy Institute.  "When there are life and death situations that are at stake you don't want to make any mistakes. ... But this area of work is really filled with the potential for failure"

Mattingly will return to a position at the Annie E. Casey Foundation in Baltimore and it’s unclear who will replace him.

With reporting by Stephen Nessen