Kate, 32, promised to guide her friend's son in his spiritual life at an intimate service in Essex on Sunday.
More than 50 people packed into the tiny picturesque St Barnabas Church in Mayland, Chelmsford, for the service, where ordinary parishioners were stunned to see Prince William, Kate and baby George take their seats.
Reverend Ken Dunstan told the Mirror: "I think my usual congregation were surprised at who was joining us.
"My main concern was that I got the tone right for the family and for the baptism.
"I don't normally pay a huge amount of attention to godparents as long as the godparents are there and willing to make the appropriate promises."
He added: "She was there as a godparent not as a princess."
Kate was one of four godparents who stepped forward during the service and promised to support her godson throughout his life. And her own seven-month-old son was very well-behaved throughout the service.
Reverend Dunstan said: "There were several babies present and all were very well-behaved."
He added: "The couple said they wanted four godparents but weren't quite sure whether one would be available.
"Then later the father rang me to say the other godmother is available, it's a friend from uni. Then he told me who it was.
"I got the impression that a number of the congregation had been to the same university."
Kate and William chose one of their best university friends, Oliver Baker, to be godfather at Prince George's christening last October.
The prince was christened by the Archbishop of Canterbury in a private service at the Chapel Royal in St James' Palace attended only by his immediate family and seven godparents.