"I was just like 'is someone messing with me?' But then as we got into further detail (I) was like, 'this is like genuine'," the 12-year-old said on Wednesday. "I didn't know what to say. I was speechless."
Last month Harry, fifth-in-line to the British throne, and actress Markle said they would invite 1,200 members of the public into the grounds of Windsor Castle, Queen Elizabeth's home west of London, where they are due to marry on May 19.
Among them would be young people who had shown leadership or served their community, Harry's office said.
Thompson's mother Lisa Newton said she nominated her daughter who was in the crowd targeted by a suicide bomber at the end of a concert by singer Ariana Grande at the Manchester Arena in northern England last May.
"She has been through a lot and she has had a obviously a rough couple of years and since Manchester it has been really hard," Newton said.
Newton herself also received an invitation but she passed it to a woman whose granddaughter was killed in the attack.
"It was something Amelia wanted to do. It feels right and it is going to make another family happy," Newton said.
On Tuesday, it was revealed that neither British Prime Minister Theresa May nor U.S. President Donald Trump would be among the 600 people invited to attend the wedding ceremony itself at Windsor Castle's St George's Chapel and a reception.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)