The two Citigate arms won the three-year account after a four-way pitch that included the eight-year incumbent team of Good Relations and Financial Dynamics.
CDR managing director Deborah Saw will lead a 20-strong team to promote the FTSE 250 company's corporate image to audiences including business, investors, the media and the Government.
CPA executive director Simon Nayyer and account director Paul Lehmann will head the public affairs team.
Securicor's three product areas include the movement of cash for supermarkets and banks, general security services and the group's justice division, which concentrates on work within the UK penal system.
Securicor is keen to widen the range of its work with the Government, which currently comprises prison management, prisoner transportation and the growing sector of electronic prisoner tagging.
Citigate's key task is to improve the promotion of Securicor's core image as a security specialist. It is understood that senior figures in the firm feel that Securicor is still too heavily linked with the telecommunications sector, despite having sold its 40 per cent stake in Cellnet to BT in 1999.
Securicor group director of communications Debbie McGrath said: 'We are asking Citigate to help represent this new focus to our different audiences.'
As well as the UK work, Citigate will assist corporate and Government affairs teams in the US, Canada, South Africa, Hong Kong, Germany and the Netherlands.
Securicor operates in 40 countries and employs around 100,000 people.