Pay, previously the managing partner, head of creative development, will join Mel Exon, the managing director, and joint chief strategy officers Jason Gonsalves and Jonathan Bottomley on a new senior management team.
The four will focus on the day-to-day running of the agency.
Ben Fennell, the agency’s chief executive, and Nick Gill, the executive creative director, will continue to focus on senior clients and on diversifying and scaling the agency’s offering.
Fennell said: "This management team has a highly complementary set of skills, but a completely aligned vision for BBH.
"Mel, Caroline, Jason and Jonathan are all outstanding individual leaders. I am excited to see what they will achieve as a team."
Pay joined BBH in January 2014 as a managing partner. She heads up the global Bailey’s account, and will launch a new global campaign for the brand this month.
She also runs the accounts for Google and Refuge. She recently persuaded ITV show Loose Women to run a domestic violence special, in which the presenter Jamelia spoke to camera about her past experience of an abusive relationship.
Pay originally worked at the agency between 2005 and 2007 when she was creative director on the Levi’s account.
So far this year BBH has launched a sports business led by Lawrence Dallaglio and Ewan Patterson and a CRM business founded in partnership with Warren Moore and Simon Hall.
BBH is planning to launch a high volume, high-speed video production business in the New Year.