The British comedian was touring his Little Britain stage show with his writing partner Matt Lucas when they were told Moss had attended a performance with her then-boyfriend, rocker Pete Doherty.
Walliams was keen to meet the blonde beauty after the show, but he kept her waiting backstage after realising MCCartney was also at the venue.
He tells Event magazine, "One night Matt and I were doing Little Britain live. We heard MCCartney was in the audience. And Kate Moss. It's one thing meeting someone at an event, but for someone like that to come and see you... You don't think people like that actually watch Tv.
"He came backstage and was really sweet and it was this amazing golden moment. But we had to keep Kate Moss and Pete Doherty waiting. We had to make a decision. Who did we think was more important and we went, 'The Beatle one!' He has to come first. We are not going to say, 'We're sorry Paul, Kate's here.'"