The young pair were joined today by Spanish nanny Maria Borallo as they watched the display. She and her son, Prince Charles, presented a new Queen's Colour to the RAF at Buckingham Palace once the service was complete.
The Choir of Her Majesty's Chapel Royal performed This is the day which the Lord hath made, which was composed for The Duke and Duchess of Cambridge'swedding in 2011, and Suo Gan, a traditional Welsh lullaby.
"I'm delighted and privileged to be christening Prince Louis today - a precious child made in God's image, just as we all are", Archbishop of Canterbury Justin Welby said on Twitter.
The Duke of Edinburgh, 97, has retired from public duties. The "Quantico" actress wore a pastel shade of purple with a matching hat by Philip Treacy, while Spencer opted for a navy blue, collared dress with polka dots and a matching hat.
Following the 40-minute service, The Duke and Duchess of Cambridge gave a private tea at ClarenceHouse. And Louis' non-royal godparents will have influence on the little prince. His daughter, Florence, was a bridesmaid at the Duke and Duchess of Sussex's royal wedding in May.
They include Guy Pelly, once dubbed the "court jester" to William and Harry for his wild ways that were often cited as a bad influence on the brothers in their youth, running their favourite drinking haunts.
Harry Aubrey-Fletcher: He went to Ludgrove School and Eton with William and his father is the Lord Lieutenant of Buckinghamshire.