Boy joined H&K in February this year from a previous position as associate director at Firefly (PRWeek, 17 February).
She took charge of H&K's 20-strong technology practice, reporting to head of corporate Tarquin Henderson.
Henderson said Boy's departure was 'agreed mutually'. He declined to comment on whether the agency was seeking a replacement.
Boy arrived at H&K to replace Lee Brooke, who left the agency to become MD at Bell Pottinger-owned technology shop Harvard in November 2008.