Elaine Lewis, mum to 20-year-old Anthony who has rare Williams Syndrome leaving him with a mental age of six, started the campaign after he was turned away from Manchester’s Trafford Centre attraction with his carer.
Staff cited their “no entry without children’” policy which they say is for “child protection”.
Since the story emerged on Friday, Elaine who lives with Anthony in Gatley, Stockport, has received messages of support from across the world.
She told the Manchester Evening News: “I’ve had comments from America and Australia and it’s not just people connected with disability, it’s adults who like Lego and are horrified they are considered a risk to children.
“So many people have signed the petition the response is incredible.
“The very fact this has got so many people talking is fantastic because disabled adults are side-lined like this all the time.”
Separately, Simon Thomason, 40, from Irlam, Manchester, who has autism and a mental age of seven, was told by Legoland the annual pass bought for him by his family will not be renewed.
Since then 2,447 people have signed Elaine’s petition calling for all adults with learning disabilities to be allowed entry when children are there.
The campaign also backed by parents’ group Autism Friendly UK.
Spokeswoman Danielle Rossato-Barker said: “Individuals with autism do not socialise and interact like people who have no learning difficulties.
Many people on the spectrum have focused interests, and a common one is lego.
“An autistic adult with the mental age of a small child who was to attend an adult-only Lego event may feel out of place.
“It should be about choice and equality.”
Both Anthony and Simon had tried to visit Legoland during the day with registered carers.
Legoland says its policy prohibits any adults without children entering apart from on monthly “adult only” evenings.
They offered Anthony a free guided tour instead of a regular daytime visit, something Elaine has described as “offensive”.
A spokesman for the venue defended the policy, saying it was “the best way to maintain a welcoming environment for young visitors”.