It follows the earlier appointments of five new members of staff this year, including Eaon Pritchard as head of digital.
Ismail joins from TBWA\Raad in Dubai where he was charged with planning for Limitless, the global real estate arm of Dubai World.
He worked on the launch of the global corporate campaign as well as launching Dubai's ambitious real estate project, the Arabian Canal.
Ismail is the co-founder of Adgrads, the online community that supports graduates looking to work in advertising, and has also worked at General Motors where he was digital marketing specialist.
In his new role Ismail will specialise in planning and creative, working across all Geronimo clients.
The appointment will support Geronimo's influx of new clients this year including ING Direct, BBC, GlaxoSmithKline, RBS Your Bank, and the growth of retained accounts, Direct Line, Seat and Radox.
Pritchard said: "This has been an extremely successful year for Geronimo and as such we are constantly striving to expand and increase our experience and diversity so that we can continue producing winning results for our clients.
"Sam lives and breathes digital and his global experience will be a valuable addition to the agency."
Ismail said: "I have known Geronimo as an agency for a while now and have always been impressed with the work they produce and the clients they work with.
"The level of experience within the agency and the emphasis on delivering results across all channels makes this a really exciting opportunity for me."