Mabey & Johnson tried to influence officials in Jamaica and Ghana when bidding for public contracts.
It also paid more than $200,000 (£123,000) to Saddam Hussein’s Iraq regime, violating the terms of the UN oil for food programme.
The Reading-based firm, which builds temporary bridges, said it regretted its past conduct.
Mabey & Johnson pleaded guilty to ten charges of corruption and sanctions violation at Westminster Magistrates Court.
The case was brought by the UK’s Serious Fraud Office.
The successful prosecution is the first of its kind against a British company operating overseas.
“These are serious offences and it is significant that Mabey & Johnson has cooperated with us to get to this landmark point,” said SFO director Richard Alderman.
Mabey & Johnson said that it had brought the matter to the attention of the SFO itself, following an internal investigation.
Commenting on Friday’s proceedings, the company’s managing director Peter Lloyd said: “We deeply regret the past conduct of our company and we have committed to making a fresh start, wiping the slate clean of these offences.”
Five of Mabey & Johnson’s eight directors have resigned since the allegations came to light.
The company is likely to face a substantial cash fine when it returns to court for sentencing.