Following the service, where Prince Louis was flanked by six godparents, the royals made their way to Clarence House where they enjoyed an intimate afternoon tea with guests.
Prince Louis was born on April 23 and is a younger brother to Prince George, 4, and Princess Charlotte, 3.
Britain's Princess Charlotte of Cambridge and Britain's Prince George of Cambridge hold hands with their father, Britain's Prince William, Duke of Cambridge, as Britain's Prince Louis of Cambridge is carried by his mother, Britain's Catherine, Duchess of Cambridge.More news: Economic Preview: There's only one dark cloud over the USA economy today
The Queen, 92, and the Duke of Edinburgh, 97, did not attend, in what was described as a "mutual decision" made some time ago.
But it was in such fragile shape that the queen commissioned her dressmaker, Angela Kelly, to make a replica.
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.
All the royal ladies in attendance-including the Duchesses of Cornwall, Cambridge and Sussex, as well as a couple of stray Middletons-stepped out wearing their Sunday best, and there were several stand-out looks. They included old school friends and a Middleton cousin.More news: Duke and Duchess of Sussex arrive in Ireland on first worldwide trip