Meghan, 36, is passing another royal milestone, days after her first balcony appearance on Buckingham Palace on Saturday, during Trooping the Colour.
She and the Queen will cut the ribbon to open the bridge over the Mersey and unveil a plaque to commemorate the event.
This is her first event without her husband, Prince Harry, by her side.
The Queen and Meghan arrived by train before travelling by car to the bridge, a £1.86bn project, which should ease traffic between Runcorn and WIdnes.
After the unveiling and ribbon cutting, Meghan and the Queen will watch a performance by local school children called Bringing Communities Together.