The firm, which was hit with 10 enforcement notices by the Health and Safety Executive last year, pleaded guilty to breaching health and safety legislation following the incident in central London in summer 2005.
In July that year, a worker was hit by a lamppost which fell on his leg after being hit by a mobile crane in Westminster. The post fell with such force that the worker had to have his foot amputated.
HSE inspector Monica Babb said: “It is essential that crane installation is properly planned and implemented.”
McAlpine was also ordered to pay costs of £12,500 at a sentencing hearing at the Old Bailey in London.