She said: "I'd like to inspire others and empower others to believe in themselves, especially people with Down's syndrome".
During the ceremony, Thompson, 59, looked beyond proud to receive the honor from Prince William.
The Oscar, Golden Globe and Emmy-winning actress and writer received the title of Dame on Wednesday from Prince William, and managed to fluster the Duke of Cambridge in the process.
"I love Prince William", Thompson said after the ceremony.
"I love Prince William, I've known him since he was little, and we just sniggered at each other", Thompson explained.
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Thompson is a longtime friend of Prince Charles and the royal family.
In a Phoebe Philo-inspired move, the actress teamed the ensemble with a pair of box-fresh trainers - a fashion move we would love to hear Her Majesty's thoughts on.
In June, the Thompson was named for the honor among a group of British actors and artists, including Tom Hardy and Keira Knightley, but revealed she had some reservations about the historical implications about accepting the award.
"It's really lovely because I've always loved the boys and I've always been a long-term correspondent with their dad, it's a very lovely feeling", said Thompson.
Thompson also said she plans to use the honor to raise awareness about schoolchildren who don't get enough to eat during school vacations, the AP reported.
She posed alongside her husband Greg Wise outside of the palace to commemorate the occasion.