Ruby McGregor-Smith was chief executive of facilities management company Mitie from 2007 to 2016. She is a Conservative Party member of the House of Lords, appointed in 2015.
The Thames Tideway tunnel, now under construction, will run for 25km under the capital, from Acton in west London to Stratford in east London, stopping millions of tonnes of raw sewage polluting the Thames. The project is currently on track for completion in 2024.
Chairman Sir Neville Simms said: "We are delighted with this most recent appointment to the Tideway board. Ruby has a wealth of experience in the commercial sector and will be an asset to the organisation as we reach the peak of construction on the project."