He was honoured 21 years after performing on the royal residence’s roof.
He played God Save the Queen for late monarch Elizabeth’s Golden Jubilee.
May — famed for hits including We Will Rock You and Bohemian Rhapsody — was recognised for his services to music and charity.
The 75-year-old rocker said afterwards: “I’m very happy, smiling from ear to ear, particularly because it was the King, which means a lot.
“We discussed the fact that we are of an age and he was wondering if my knees were holding out for the kneeling part of the ceremony.
“I said: ‘Yes, just about’.”
Sir Brian — who is also an astrophysicist and animal welfare advocate — posed with his wife, ex-EastEnders actress Anita Dobson.
And he gave Queen fans a lift when he said: “We’re thinking about touring.
“There’s been times I thought I’d never do it again. But we all seem to be OK.
“If we don’t go out until October or whatever I’ll be working on my fitness from now.”