Carried inside by his mother, it is the first time the family of five have been pictured together.
Video shared by Kensington Palace also showed grandparents Prince Charles and his wife, the Duchess of Cornwall Camilla, arriving – as well as newlyweds Harry and Meghan.
Prince Louis was christened at St James’s Palace in a private, 40-minute ceremony conducted by the Archbishop of Canterbury Justin Welby in the Chapel Royal.
The young prince, who appeared to be sleeping, wore a replica of the royal christening robe, a white lace, satin-lined dress first worn in 1841.
It had been reproduced by dressmaker to the Queen Angela Kelly and was worn by his older siblings, George and Charlotte at their christenings.
As Kate chatted to the archbishop while holding Louis, she could be heard describing her sleeping son as very relaxed and peaceful.
The duchess added, with a smile I hope he stays like this.
The Duchess of Cambridge wore a calf-length white dress, while Meghan, Duchess of Sussex, wore an olive green Ralph Lauren outfit and Stephen Jones hat.
Members of the Royal Family arrive at St James’s Palace for the christening of Prince Louis. pic.twitter.com/3pDk4D898C
Princess Charlotte wore a light blue dress and hairband.
Eleven-week-old Louis, who is fifth in line to the throne, is to have six godparents.
The Duke and Duchess of Cambridge chose two hymns, two lessons and two anthems for the christening of their son.
The Hymns were O Jesus, I Have Promised and Lord of All Hopefulness.
The lessons were from Mark 10 13-16, read by godparents Lucy Middleton and Ephesians 3 14-19, read by Guy Pelly.
The anthems were This is the day which the Lord hath made, which was composed for the Cambridges’ Wedding in 2011, and Suo Gn, a traditional Welsh lullaby.
Both were performed by The Choir of Her Majesty’s Chapel Royal.
The Queen and Duke of Edinburgh did not attend the christening however, after a busy week of high-profile events in central London and ahead of a visit by US President Donald Trump.
Kensington Palace said that the Queen’s non-attendance is not related to illness, but was mutually agreed some time ago.
The christening had been described as a small affair with immediate family and godparents.
The Duke and Duchess of Cambridge have asked Nicholas van Cutsem, Mr Pelly, Harry Aubrey-Fletcher, Lady Laura Meade, Hannah Gillingham and Miss Middleton to be godparents for their youngest child.
All are friends or family of the royal couple.
Photographs of the occasion the family are to be taken by Matt Holyoak, who photographed the Queen and Duke of Edinburgh on their 70th wedding anniversary.
Prince Louis’ birth certificate released
Water from the River Jordan is to be poured from the Lily Font, a silver baptismal font commissioned by Queen Victoria in 1840, during the baptism.
Prince Louis cradled by Princess Charlotte in first official pictures
After the ceremony, the Duke and Duchess of Cambridge were to serve guests christening cake, taken from one of the eight tiers of their seven-year-old wedding cake, at a private tea.
Who are Prince Louis’ new godparents?
Nicholas van Cutsem
An international business developer, Mr Van Cutsem is friends with both Harry and William through their parents, who met at University of Cambridge. He is a former major with the Household Cavalry’s lifeguards.
Party animal friend of Harry and William, Guy Pelly spent his career running posh nightclubs. He attended prep school and Eton College with Prince Harry, who attended his wedding in 2014.
Prince William was an usher at Mr Aubrey-Fletcher’s wedding to Louise Stourton in 2010. He went to Eton College and went on to work in mergers and acquisitions at a US banking firm.
Lady Laura Meade
Lady Meade is married to James Meade, who is one of the godparents of Princess Charlotte. The pair are friends with William and Catherine and James spoke at their wedding.
Kate’s cousin on her father’s side, Ms Middleton is a London-based solicitor. Her daughter was a bridesmaid at the wedding of Meghan and Harry in May.
Otherwise known as Mrs Robert Carter, Hannah Gillingham is one of Kate’s best friends from school – the exclusive Marlborough College in Wiltshire. The two are thought to have played on several sports teams together.
News Source SkyNews