The District government’s top internal watchdog, Inspector General Charles J. Willoughby, will retire after a nine-year tenure, Mayor Vincent C. Gray’s office announced Monday.

Willoughby’s current five-year term is set to expire May 19, and Gray (D) had made no move to reappoint him.

Under Willoughby, the Office of the Inspector General has played a role in investigating some of the most egregious examples of District government corruption in recent years, including a $50 million Office of Tax and Revenue embezzlement scandal and a rampant double-dipping of unemployment benefits among city employees. It also produced widely hailed reports examining the government’s failures surrounding the 2008 Banita Jacks tragedy and the 2006 death of New York Times reporter David Rosenbaum.

But Willoughby, who has maintained a low public profile amid a period of great turmoil in city government, has been accused in some quarters of a lack of urgency in examining several politically sensitive matters, including the award of the city’s lottery contract and allegations of widespread test cheating in the D.C. Public Schools. More recently, Willoughby has clashed with the upstart Board of Ethics and Government Accountability, which has taken more aggressive positions in examining two particular contracting controversies, surrounding D.C. Council member Jim Graham (D-Ward 1) and Chief Administrative Law Judge Mary Oates Walker.

In a letter sent to Gray on Friday, Willoughby told the mayor that it had been a “pleasure and honor” to serve the District government and its residents since his 2005 appointment. “During this period, I believe that the work of the Office of Inspector General has greatly contributed to the District achieving strides from a fiscal and operational standpoint,” he wrote. “That work and the service associated with it is something that I will always appreciate since it afforded me the opportunity of public service.”

Willoughby, 64, a former federal prosecutor who had previously served as an assistant inspector general for the Federal Communications Commission, declined to comment through a deputy.

Gray’s office issued a statement thanking Willoughby for his service. “The position of Inspector General has an important role as an independent watchdog of the District government, and I look forward to nominating a new Inspector General in the coming weeks.”