Janus, who was the firm's client services director, will be based at the organisation's Cirencester offices.
He replaces Greenhalgh who held the role since March 2008. Mike Spicer, group managing director for EHS Brann, said the agency wished Greenhalgh "all the best for the future".
"Richard has done a fantastic job in leading EHS Brann Discovery to achieve significant new business wins as well as driving best practice, gaining fantastic credentials for the business," Spice said.
"He was also instrumental in EHS Brann Discovery recently being awarded the ISO 14001 accreditation for its adherence to stringent environmental standards.
Janus worked at OgilvyOne and client-side at NTL before joining EHS Brann Discovery in May 2002.
"Olivier has been a powerful force within the agency since his arrival and I have no doubt that his experience and passion will continue to drive EHS Brann Discovery forward in delivering solid and dynamic solutions for its clients," Spicer said.