He will report to Rathi Gangappa, MediaCom India Chief Operating Officer.
Vishwanathan takes over from Sriram Sharma who is moving to another role in Mindshare, the company said in a statement.
"We are happy to have someone with Vishi's experience and calibre on board. Our business in the South has seen fantastic growth in the last few years under Sriram's leadership," MediaCom India Managing Director Debraj Tripathy said.
"I am confident Vishi will bring renewed energy and focus to our business. I wish Sriram all the best in his new role," Tripathy added.