Gilley (pictured) will replace Sara Collinge, who moved to consultancy Don't Cry Wolf as managing director in October.
Gilley, who takes her new role at the tech PR specialist in January, previously spent 19 years at Bite Global. Her departure follows the announcement in August that Bite Global and sister agency Text100 are to merge.
Clarity CEO and founder Sami McCabe said: "To attract someone of Rachel’s profile and pedigree to join Clarity is a huge coup. It’s both a recognition of the strength of our position today, and a validation of our vision for the future of the agency. Rachel will doubtless contribute significantly to delivering against our ambitious growth plans – both locally and globally."
Gilley said: "Clarity’s vision for the agency of the future is extremely exciting, and I’m looking forward to working closely with the global leadership team to make it a reality."
Clarity generated global revenue of $3.4m in 2017, employing 26 staff, according to PRWeek's Global Agency Business Report. The London-headquartered agency also has offices in Berlin, New York and most recently San Francisco via the acquisition of DRSmedia earlier this year