The shadow chancellor adorned a treat to honour the Monarch's official birthday for ITV's 'Peston on Sunday' programme, but given the decoration, might have done so begrudgingly.
McDonnell, who in January made headlines amid speculation he would refuse to kneel before Queen Elizabeth II at his swearing in to the Privy Council, today went for a pink icing border and the simple number '90'.
ITV's political editor Robert Peston showcased the finished baked product.
He was also pictured in a Bake Off-style preparation shot squeezing yellow icing out of a tube.
In his appearance later in the programme, McDonnell urged Labour supporters to vote to stay in the European Union as a “Tory Brexit” would be much worse.
He said the Labour argument had been “squeezed out” by the media’s focus on the “punch-up” in the Tory party involving David Cameron and Boris Johnson.
Other guests also invited on to the Sunday morning politics programme invited to decorate a cake for the Queen included Ukip's Suzanne Evans, who said she was in "heaven".
Comedian Omid Djalili and Scottish Conservative leader Ruth Davidson also appeared in the studio to ice cakes for the monarch.