The Special Inspector General for Afghanistan Reconstruction (SIGAR), the watchdog agency charged with monitoring the more than $70 billion the U.S. has committed to rebuilding Afghanistan since 2001, has seen its share of ups and downs.

Earlier this year, Arnold Fields, the former head of SIGAR, stepped down after coming under serious criticism for his management style and the agency’s lackluster performance.

Now Herbert Richardson, the acting inspector general, is making an exit.

“After more than 37 years of public service, I’ve decided to accept an opportunity in the private sector, at a time when I’m convinced SIGAR has changed course, is producing results, and is being led effectively by the new leadership team that I’ve put in place,” Mr. Richardson said in a statement.

In recent months, the agency had probed the failures of both Afghan and U.S. agencies to tighten controls over Afghanistan’s financial sector. It also pointed to major weaknesses in the way the U.S. and its allies are building up the Afghan police.

In an interview, Mr. Richardson said the agency had shifted emphasis in recent months “following the money,” with auditors and criminal investigators putting more effort into contract investigations.

Mr. Richardson departs on Sept. 2.