When the real Queen was unavailable to meet with Oliver Burton, 10, who has Down syndrome and was recently diagnosed for a third time with cancer, Mirren invited the boy and his family to see her in the Peter Morgan play "The Audience," in which she portrays Elizabeth II.
The National / Regional Children's Tumour Leukaemia Cancer Trust, which arranged the visit, ran Facebook photos of the Oscar winner backstage with the boy, who is from Leicester in England's East Midlands.
The organization thanked Mirren and others for granting the boy's "most wanted" wish.
Oliver and his family were brought by limo to the Gielgud Theatre in London's West End -- where after the play he and his mother, Catherine, met Mirren backstage, as a butler served refreshments and two corgis romped.
"She stayed in character for the whole thing," the boy's father, James, told British media. "Oliver thought she was the real Queen, and well, that's good enough for us. She was really lovely. . . . She sat in Oliver's wheelchair and gave him her big chair."
He added, "She knighted him and told everyone that they had to call him Sir Oliver.
"He had a brilliant day."
Mirren told the UK newspaper The Sun, "It was a pleasure and a privilege to meet such a brave young man."