The closest airport to Cambridge is Stansted Airport which is only a thirty minute bus or train journey away.
You can get to Cambridge from London by rail (services run regularly night and day). With trains running every 15 minutes from both Liverpool Street and King's Cross stations. The journey takes from 45 to 90 minutes dependent on the train.
Regular buses to Cambridge's Drummer Street bus station depart every hour from all major airports, as well as National Express coaches to and from all major cities in Britain.
Useful websites to plan your travel: