Oliver Dowden joined the agency in January as managing consultant, reporting to public affairs MD Tim Fallon.
Dowden had previously been Tory deputy campaigns director, working closely with Conservative leader David Cameron.
He spent three years at Conservative HQ. Before this, he was as an account director at lobbying firm LLM.
He is understood to be returning to Tory HQ as deputy director, political operations, reporting directly to comms director Andy Coulson.
The departure of Dowden is a huge blow for H&K. Earlier this year, PRWeek named Dowden as one of eleven PR professionals with especially strong links to the current Conservative leadership.
H&K's Fallon put a brave face on the news. He said: 'Whilst we are disappointed to lose Oliver he goes with our blessing. This is a great opportunity for him at what is a unique and exciting time in British politics.'