Her appointment coincided with Tideway publishing its 2018/19 annual report confirming the project is on time to be delivered by 2024.
The report also confirmed the first 3km of the super sewer is now complete underneath the River Thames.
Chairman Sir Neville Simms, formerly charman of Carillion until 2005, 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.”
Now a member of the House of Lords, McGregor-Smith was chief executive of facilities management contractor Mitie from 2007 to 2016.
Earlier this year Tideway announced that it had increased its project cost estimate by 8%.
This included substantial investment in the company’s river transport plans and unforeseen engineering challenges.
Tideway confirmed that there will be no change to the estimated £20-25 annual cost range for Thames Water bill-payers.
Within its annual report Tideway also revealed details of its innovative river transport plan to reduce the project’s carbon footprint and take pressure off London’s road network.
It is taking around 200 lorry journeys off the road each day by using the river to transport material.
More than one million tonnes of material have now been transported by river, saving more than 115,000 HGV movements.
Trials undertaken by Tideway have shown using the river can produce significantly fewer emissions, with a 1,000 tonne barge producing an average of 90% less carbon dioxide than a standard HGV equivalent.
Andy Mitchell, Tideway CEO, said: “With work on the tunnel in full swing, and a continued focus on keeping people safe, our operations this summer will see a major increase in our river traffic.”