Showrunner Peter Morgan has also revealed the series will end after season five, which will see Imelda step into the role as Her Majesty.
"At the outset, I had imagined The Crown running for six seasons, but now that we have begun work on the stories for season five, it has become clear to me that this is the perfect time and place to stop. I'm grateful to Netflix and Sony for supporting me in this decision."
It sounds like Imelda is thrilled to have landed the role!
"I have loved watching The Crown from the very start," she said in a statement posted to the show's Twitter account. "As an actor, it was a joy to see how both Claire Foy and Olivia Colman brought something special and unique to Peter Morgan's scripts. I am genuinely honoured to be joining such an exceptional creative team and to be taking The Crown to its conclusion."
Late last year, reports circulated that Imelda was in talks to take over the role from Olivia, but Netflix dismissed them as "pure speculation."
Imelda has an impressive acting resume that will surely be a huge boon to the series. The BAFTA-winning star is known for her role as Dolores Umbridge in the Harry Potter films and for playing Lady Maud Bagshaw in the Downton Abbey movie.
Imelda also has experience playing royals, so this casting makes a lot of sense. In the BBC TV drama Cambridge Spies, set in the UK from 1934 to 1951, she played the Queen Mother, then known as Queen Elizabeth during the time of King George VI’s reign.
Season three of The Crown debuted in November and sees Olivia pick up where Claire left off. Season four is currently being filmed, and will cover the rise of late former U.K. Prime Minister Margaret Thatcher (Gillian Anderson) and Prince Charles's marriage to Princess Diana (Emma Corrin).
It remains to be seen what season five will cover, but it's likely it would touch on events such as the childhoods of Prince William and Prince Harry and the deaths of Princess Diana, Princess Margaret and the Queen Mother.