The PM was speaking at the FCO Leadership Conference which took place in London on 6 July.
The Conference brings together over 200 British Ambassadors, High Commissioners and senior diplomats once a year.
The PM also called on diplomats to use their skills and contacts to become “economic ambassadors for Britain” to help build our export-led growth that will “secure our influence and earn our living in a rapidly changing world”.
The PM said:
I want you to ask yourself every day: ‘What am I doing to promote British business?’
If you want to keep Britain’s great ambassadorial residences - then I want you to show me that every day you are using them relentlessly to open new trade links and to generate new business for Britain.
Mr Cameron also said he wanted the FCO to be open and engaged with the world as well as being “hard-headed and practical”.
Deputy Prime Minister Nick Clegg, who also spoke at the Conference said that in these “new and challenging times” we need to “reassert ourselves in the world”.
All of the big issues are international by nature: from better regulating the banks, to stimulating growth in the economy, to tackling climate change and combating terrorism and cross-border crime.
That means a new approach to foreign policy, trade, the economy and security, where we find and implement solutions internationally, led by our first class Foreign Office.