Olivia Birchenough and Jen Pringle from Channel Five's kids show Milkshake! cut the ribbon outside the £1.5m centre, which is located at Albert Dock.
Housed within the Grand Hall, the venue includes three immersive zones that feature characters and scenery from Thomas and Friends, Bob the Builder and Fireman Sam.
There is also a café, retail space, dedicated party rooms and a green screen experience for children to enjoy.
The venue, which spans 13,000 sqft, is owned and operated by Heritage Great Britain, the firm behind tourist attractions such as Land’s End and Snowdon Mountain Railway.
Graham Dentith, general manager of Mattel Play! Liverpool, said: "We’re so pleased and excited to be finally opening our doors to families. I hope everyone will get as much fun and enjoyment from visiting Mattel Play! Liverpool as we had from creating it."
Allan Leech, CEO of Heritage Great Britain, added : "After months of planning and preparation, we’re delighted to reveal this new concept which is a first for Liverpool.
"Everyone’s pulled together to make this happen and we’re grateful to all our city partners, local contractors and suppliers, and to Lappset Creative who built the attraction."
Back in 2014, Mattel's entertainment company HIT Entertainment, created a Thomas and Friends play zone at Lollibop festival in Hatfield House, Hertfordshire.
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