The service to receive former Prime Minister Tony Blair into the Roman Catholic Church was "moving", Cardinal Cormac Murphy-O'Connor has said.
Tony Blair visited Pope Benedict XVI at the Vatican in June
The leader of Catholics in England and Wales said Friday's service - attended by Mr Blair's wife and four children - was "very intimate, very prayerful".
The Vatican has welcomed Tony Blair's decision to become a Catholic.
Mr Blair, who had been an Anglican, became a Catholic during a service at Archbishop's House, in Westminster.
Cardinal Murphy-O'Connor, who is the Archbishop of Westminster, led the service and told BBC Radio 2's Good Morning Sunday: "It was a very moving occasion.
"I suppose for him [it was] the end of a process, in the sense that he's been thinking about becoming a Catholic for a long time.
"But also, in another sense it's a beginning, because when you become a Catholic, as so many people who have become Catholics have said to me, it's like coming home.
"This was a gift for Tony, a personal journey, a gift for his family."
Cardinal Murphy-O'Connor said Mr Blair, who was formerly an Anglican, was becoming a Catholic "partly because... the example of his family, his children have been brought up as Catholics.
"I think also it's not just in his travels as prime minister, but even before that, there was something he said to me, that he feels at home in the Catholic Church in a way that he didn't in any other Church, or in the Anglican communion."
Cardinal Murphy-O'Connor led Mr Blair's service
A Vatican spokesman said such an "authoritative personality" choosing to join the Catholic Church "could only give rise to joy and respect".
Last year, Mr Blair, who is now a Middle East peace envoy, said he had prayed to God when deciding whether or not to send UK troops into Iraq.
And one of Mr Blair's final official trips while prime minister was a visit to the Vatican in June where he met Pope Benedict XVI.
BBC correspondent David Willey said it had been no secret in Rome that Mr Blair had been taking instruction from a Catholic priest as a prelude to conversion.
The Archbishop of Canterbury Dr Rowan Williams, leader of the Anglican church, wished the former prime minister well in his spiritual journey.
But ex-Tory minister Ann Widdecombe - herself a Catholic convert - said Mr Blair's voting record as an MP had often "gone against Church teaching" and that his conversion raised some questions.
"If you look at Tony Blair's voting record in the House of Commons, he's gone against Church teaching on more than one occasion. On things, for example, like abortion," she said. "My question would be, 'has he changed his mind on that?'"
There has never been a Roman Catholic prime minister of Britain, although there is no constitutional barrier to such a move.
However, it had in the past been suggested that Mr Blair would wait until after leaving office, to avoid possible clashes such as over his role in appointing Church of England bishops.
His conversion comes as new research shows that for the first time in decades, the number of Roman Catholics attending Sunday services may fall behind the number of Anglicans doing so.
When the organisation Christian Research began its census of church attendance in England in 1979, 1,991,000 Catholics attended Sunday mass compared with 1,671,000 Anglicans.
But estimates for worshippers in 2006 showed 861,800 Catholics attended Sunday services, compared with 852,500 Anglicans.
Benita Hewitt, executive director of Christian Research, said that by 2010 predictions showed there would be more Anglicans attending on Sunday than Catholics.