The British actress said she was thrilled to be given the honour - and joked that her star being adjacent to Colin Firth's was the cherry on the cake.
"I couldn't be prouder and more happy that I'm actually going to finally lie next to Colin Firth, something I've been wanting to do for a very long time," laughed the 67-year-old Oscar-winner.
Helen clinched her Academy Award for her portrayal of the Queen in 2006, while Colin won his for playing the monarch in The King's Speech in 2010.
The actress, who was joined by her husband Taylor Hackford and her children at the star ceremony, said: "Well I'm very pleased and proud and I think it's very good for the British monarchy that here on Hollywood Boulevard, the King and the Queen are going to actually sleep together, for the rest of history."
Helen, who has also scooped several Baftas, said it was a "spectacular day".
"I really can't believe it. It is amazing," she added. "I am going to have to come here tomorrow and have another look at it, when everybody is gone. People are just coming and going, I will be in disguise just standing here, leaning up against there."
Writer and director David Mamet was among those at the event. He told the crowd: "Helen's performances reap higher praise than being praised, they're loved."
She is nominated for a Golden Globe for her latest work in the film Hitchcock, which explores the relationship of influential film-maker Alfred Hitchcock and wife Alma Reville during the filming of Psycho in 1959.