Robb, who has led the OTS since 2006, will take over from task force director Naomi Eisenstadt, who retires at the end of this month. The position was not advertised.
A Cabinet Office spokesman denied Robb's dual role would water down his focus on third sector issues.
"It means the task force and the OTS will be able to work better together," he said.
The task force, which coordinates the Government's drive against social exclusion, is based one floor away from the OTS at the Cabinet Office.
Robb, who was director of public policy at third sector umbrella body the NCVO before joining the civil service, will divide his time flexibly between the posts rather than dedicate a set amount of time to each job.
He said he was excited about his new role.
"Both of these units require not only strong leadership across Whitehall but also extensive networks outside government," said Robb. "I hope the positive relationships I already have will be of mutual benefit to both units.
Third sector minister Angela Smith is also the minister responsible for social exclusion.