The duke and duchess were in direct competition at Twickenham Stadium for what was the first match Kate attended since becoming patron of the Rugby Football Union (RFU).
William, who has been patron of the Welsh Rugby Union (WRU) since 2016, and Kate are known for their rivalry during sporting engagements and the pair sported their own team scarves on Saturday.
George sat in between his mother and father at the game, and Kate was seen leaning down to talk to her smiley eight-year-old son, while both parents were seen to point things out to him.
The young prince was wearing a navy and red coat, matching his father’s choice of colours, while Kate wore a black and white houndstooth jacket.
Kate will be pleased with the final scoreline which was 23-19 in England’s favour.
Prior to kick-off, the royals were expected to meet Jeff Blackett, president of the RFU, and Gerald Davies, president of the WRU.
The duchess, known for her love of sport, said earlier this month she was “very proud” to become the RFU’s figurehead, a role that used to belong to the Duke of Sussex.
Kate, 40, grew up watching England rugby games with the Middletons, and the family would plan their weekends around international matches.
At an England training session earlier this month, she laughed when player Ellis Genge asked her about the new rivalry with William and admitted it would “make things interesting”.
Kate said: “Ha, yes, what does that mean? We’re terribly competitive.
“The fact we’re now supporting two different teams is going to make things interesting.”
The duchess also told of her children’s enthusiasm for rugby, saying George plays the sport at school and three-year-old Louis likes to be in the “middle of everything”.
She is the first member of the royal family to officially receive one of the Sussexes’ past royal patronages, which they were stripped of post-Megxit.
