It was the biggest day of his short life so far, surrounded by his loving family and doting godparents for the historic moment of his christening.
Prince Louis, who snoozed peacefully through proceedings, appeared to know he was in safe hands.
The serene Prince, just 11 weeks old, was "very relaxed and peaceful", his mother said, as she told the Archbishop of Canterbury: “I hope he stays like this”.
The Duchess of Cambridge, who cradled her youngest son in his lace and satin dress, was joined by the Duke for the 40-minute-long ceremony, with siblings Prince George and Princess Charlotte holding his hand tightly.
Six godparents included the Duchess’ cousin, solicitor Lucy Middleton, a handful of the Duke’s schoolfriends and reformed “court jester” Guy Pelly attended as godparents.
Members of the Royal Family arrive at St James's Palace for the christening of Prince Louis. pic.twitter.com/3pDk4D898C— Kensington Palace (@KensingtonRoyal) July 9, 2018
Mr Pelly, who attended the christening with his wife Lizzy, is perhaps best known for accompanying Princes William and Harry in livelier days on the London nightclub scene, and has previously said he is now “leaving it to the next generation”.
The Queen, 92, and the Duke of Edinburgh, 97, did not attend the christening, in what was described as a “mutual decision” made some time ago.
It was not due to ill-health on the part of the Queen, a source said, with Her Majesty having a busy schedule in Scotland last week and London in the coming days, including the RAF centenary on Tuesday and a visit from Donald Trump.
They are understood to have also missed the christening of their great-granddaughter Isla Phillips. Proud grandfather the Prince of Wales and Duchess of Cornwall, along with the Duke and Duchess of Sussex who walked hand-in-hand.
The small christening party saw Prince George and Princess Charlotte politely shake hands with the Archbishop of Canterbury and Dean of Windsor, praised by their father who told them “well done”.
The Duchess was heard to remark that the outing “reminds me so much of George”, who was also christened there.
As they left, the three-year-old Princess clutched an order of service as a souvenir, while her four-year-old brother peered calmly at his baby brother.
It is the first time the Cambridges have been photographed together as a family of five, with Prince Louis considered too young to join them for previous events including Trooping the Colour.
They were joined by grandparents Michael and Carole Middleton, uncle James Middleton and pregnant aunt Pippa, who attended with her husband James Matthews.
Godparents in attendance included Mrs. Robert Carter, nee Hannah Gillingham, a school friend of the Duchess, and Lady Laura Meade, who is married to Prince William's close friend and Princess Charlotte’s godfather, James.
Nicholas Van Cutsem is an old family friend whose daughter Florence was a bridesmaid at the wedding of the Duke and Duchess of Sussex, while Harry Aubrey-Fletcher went to Ludgrove School and Eton with the young Prince William.
Mr Pelly,who has been a loyal friend to both Prince William and Prince Harry despite a reputation for encouraging them astray, has previously run night spots including Mahiki and Whisky Mist.
He was once mistakenly tipped to be a godfather to Prince George in 2013, saying then: “I love babies and I have calmed down quite a bit socially now. I think I will be a very, very good godfather - one day, at least.”
Mr Pelly, who married Elizabeth Wilson in 2014, will now have his chance, trusted with a reading of Ephesians 3: 14-19 at the christening.
Prince Louis godparents
Lucy Middleton read St. Mark 10: 13-16, before the small congregation sang O Jesus, I Have Promised and Lord of All Hopefulness.
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’s wedding in 2011, and Suo Gân, a traditional Welsh lullaby.
The Duchess of Cambridge wore a dress by McQueen and hat by Jane Taylor, in a cream shade similar to those she chose for the christening of her two older children, while the Duchess of Sussex wore olive green Ralph Lauren.
Official photographs, taken by Matt Holyoak who was also entrusted with pictures from the Queen and Duke of Edinburgh’s 70th anniversary, will be released later this week.
After the 40 minute service, the party re-emerged, the Duchess of Cornwall fanning herself with an order of service to relieve the stifling heat before the family headed back to the palace for tea and a slice of fruit cake saved from the wedding of Prince William and Kate Middleton.
The robe is a replica of the intricate lace and satin dress made for Queen Victoria's eldest daughter in 1841, and was also worn by George and Charlotte.
Prince Louis, in pictures
Key moments to look out for at the christening
Prince Louis will be christened behind closed doors in a private ceremony.
Here are some things to look out for on Prince Louis's big day:
The star of the show
As he is taken to and from the service, it will be the first time Louis has been seen - other than in photographs released by Kensington Palace - since making his public debut on the steps of the Lindo Wing.
Prince George gurgled and blew bubbles ahead of his christening in 2013, while Princess Charlotte was seen crying as she was about to be taken into church in 2015.
Charlotte - who is three - is becoming known for her polished public appearances where she confidently gives a royal wave for the cameras.
Royal christenings through the years, in pictures
Will Prince George get bored?
George was spotted looking less than enthused at the Trooping the Colour celebrations in 2017.
George, who is nearly five, is expected to join his family for the 40-minute church service, but royal nanny Maria Teresa Turrion Borrallo could be on hand in her traditional Norland uniform to look after her charges.
William and Kate's choice of godparents offers an insight into their trusted inner circle.
In the past, the Duke and Duchess have chosen close friends and confidantes including their university friend Oliver Baker and Kate's school friend Sophie Carter.
The Duchess of Sussex
It will be the Duchess of Sussex's first time at a royal christening if she attends as expected.
All eyes will be on the Duchess's choice of outfit and if she continues her trend of opting for a neutral palette.
Royal fans will also be looking for signs of broodiness as she joins the Cambridges' young children at the service.
The frilly christening gown
Royal babies are baptised in the same intricate cream lace christening gown.
It is a replica of a 19th century one used for most of Queen Victoria's children which is now too delicate to wear.
At the christening of William's children, there has usually been a poignant nod to the duke's late mother Diana, Princess of Wales, who was killed in a car crash in 1997.
The princess's friend Julia Samuel was made George's godmother, while Diana's niece Laura Fellowes is one of Charlotte's.
The Chapel Royal is where Diana's coffin lay before her funeral to allow her family to pay their last respects.
Tradition of royal christenings through the years
Royal christenings are steeped in tradition, with customs passed down through the generations.
When Prince Louis is baptised on Monday at the Chapel Royal, the Duke and Duchess of Cambridge are expected to keep to convention.
Louis is expected to wear the royal christening robe - a replica of the original Honiton lace and satin robe made in 1841 for Queen Victoria's eldest daughter.
The new robe was created by the Queen's dresser Angela Kelly. The original cream lace and white satin robe was last used in 2004 and is too delicate to be worn.
According to tradition, the water used for Louis's christening will be holy water from the River Jordan, where it is said Jesus was baptised by John the Baptist.
Louis will also be expected to be christened using the ornate silver gilt Lily Font - part of the Crown Jewels.
It is decorated with lilies and ivy foliage around the rim, features three cherubs around the base, and the main bowl is a large lily bloom.
The Chapel Royal at St James's Palace, London, was used for Prince George's christening in 2013.
Royal christenings are usually private affairs and most royal babies have been christened at royal palaces.
William and the Prince of Wales were both christened in Buckingham Palace's Music Room.
The Queen, then Princess Elizabeth, was christened in the Palace's private chapel in 1926.
Prince Harry, now the Duke of Sussex, was christened in St George's Chapel, Windsor Castle, where he was married in May.
Princess Charlotte, however, was christened at the Church of St Mary Magdalene at Sandringham in Norfolk.
She was pushed to her christening past well-wishers in a vintage 1950s silver-wheeled Millson pram - once used for the Queen's youngest children Prince Andrew and Prince Edward.
What we know about new prince so far
Prince Louis of Cambridge is 11 weeks old - here is what we know about him so far:
- His full name is HRH Prince Louis Arthur Charles of Cambridge.
- He was born on patriotic St George's Day - April 23 - at 11.01am at the private Lindo Wing of St Mary's Hospital in Paddington, London.
- He weighed 8lb 7oz - but was not the heaviest royal baby in modern times. The Queen's first great-grandchild Savannah Phillips weighed 8lb 8oz in 2010, and the newest addition to the Windsors, Zara Tindall's daughter Lena, born last month, weighed 9lb 3oz.
- Louis, the Duke and Duchess of Cambridge's third child, is the Queen and the Duke of Edinburgh's sixth great-grandchild. They now have seven after the arrival of Lena Tindall.
- He slept well early on. Two days after Louis was born, William revealed at an Anzac Day service that the baby and Kate were "in good form, luckily". "Sleeping's going reasonably well so far, so he's behaving himself, which is good," he added.
- But it's three times as much worry for the Duke. Father of three William declared it was "thrice the worry now" when he left the hospital with the duchess to take newborn Louis back home to Kensington Palace.
- The name took people by surprise. William and Kate had already used Louis as one of Prince George's middle names, but liked it so much they decided to use it again. It has a poignant meaning for the Windsors. Louis was the name of the Prince of Wales's beloved great-uncle Earl Mountbatten, who was murdered by the IRA in 1979.
- Princess Charlotte is smitten with her little brother. Kensington Palace released sweet images of Charlotte cuddling baby Louis to mark her third birthday. Charlotte was pictured gently kissing the newborn on the top of his head.
- He is unlikely to be king. Louis is fifth in line to the throne, with the Prince of Wales, William, George and Charlotte ahead of him in the line of succession.