The hire marks a significant coup for APCO. Murphy has worked at Number 10 for the past three years, initially as assistant political secretary to the PM. Prior to that, he was special adviser to former Secretary of State for Health Alan Milburn for four years.
It is his understanding of the decision-making process at the Department of Health that is likely to be of particular benefit to APCO, which has traditionally had a robust portfolio of clients in the healthcare sector.
He was made special adviser to the Deputy PM John Prescott when Labour came to power in 1997.
'I never intended to see out the Prime Minister's period in office,' explained Murphy. 'This gives me a great opportunity to get some experience in the private sector.'
He said he would bring 'knowledge and understanding of how New Labour thinks and experience of crisis management'.
He was approached for the role by APCO EMEA chief executive Brad Staples, who said: 'Darren is one of a very small number of people who work with the PM every day and should be of significant value to a number of clients'.
Murphy will become one of five London-based directors at APCO when he takes up the post on 5 September, reporting to UK managing director Simon Miller.
The appointment comes as APCO announces the opening of an office in Tel Aviv, headed by Gad Ben-Ari, former media adviser to the late Israeli Prime Minister, Yitzhak Rabin.
APCO president and CEO Margery Kraus said: 'Israel is attracting foreign investment to its high-tech and science industries', adding the opening 'reflects the growing importance of Israel in the world market.'
Ben-Ari was most recently international director general of fundraising organisation Keren Hayesod-United Israel.