Although Mr Murphy, 40, takes over a post once held by Peter Mandelson, he has told friends he does not want to become Labour's new "king of spin", but to be more involved in long-term planning.
His pounds 40,000-a-year job as assistant general secretary (communications) will give him a key role in developing Labour's strategy for the next general election. He has abandoned the idea of becoming a parliamentary candidate to focus on the task.
His first big test after leaving his current post, as executive director of communications at the Arts Council, will be to run Labour's campaign in a busy election period this spring. There are elections to local authorities, the Scottish and Welsh assemblies in May, and to the European Parliament in June.
Mr Murphy said: "I regard it as a privilege. I see it as a critical job, not just for the round of elections coming up but for the longer term planning for the general election."
Mr Murphy was a Westminster journalist for 12 years, working as political editor of the Press Association and before that the Yorkshire Post. He replaces David Hill, who left Labour HQ last year to join the lobbying firm Bell Pottinger.