Talks have been underway since the start of the summer and a deal is expected to be completed this month.
The details, including equity stakes and senior management changes, have yet to emerge. Mustoes is currently run by Nick Mustoe, the managing director and founder.
It is thought that the deal is designed to reinforce Mustoes' client base and offer its clients wider access to PR and direct marketing services.
Geronimo will be able to offer above-the-line advertising as well as digital advertising through Cosmic ID, Mustoes' in-house digital arm. The merger will also allow Geronimo to move further into the corporate sector.
The move comes 18 months after Mustoes bought back an 80 per cent stake in the agency from Japanese advertising giant Hakuhodo, reclaiming independence after selling in 2002.
The deal is similar to Freud's buyout of DFGW last year, which saw the agency absorbed into the PR giant to offer clients advertising and film services.
Mustoes' clients include Kia, Bombardier and Schott's Almanac, while Geronimo handles The Department for Schools, Children and Families, and BBC Magazines.