He’s been here before. What situation has he not been in? Grand slam victories in the Six Nations, European titles, but never a World Cup.
At his age, Sexton is at his last dance and despite that number, on Saturday, he showed once more just how vital he is to the team.
Their first try scored by winger Mack Hansen was vintage Sexton. On the wrap-around, the fly-half burst through the line, only tackled on the line to stop him scoring a try. That led eventually to the score, but, for Ireland, Sexton’s organisation is not as vital as his right foot.
He kicked five important points with a perfect success rate. This is in contrast to his opposite number Manie Libbok. Despite the flair and skill of Libbok, Sexton proved his worth once more in such a tight game, the score only 13-8 on a pulsating evening in Paris where the Irish fans really made themselves heard.
With Scotland to come, now a knockout game in all but name, the Irish number 10 will once again play a major role.
But, was this foreseen given Sexton’s age? The veteran had not played for six months before their first game at this World Cup - an 82-8 win over Romania - and he looked like as good as ever. His play, never eyebrow-raising, is metronomic and reliable, always finding the man outside and always making himself available thereafter.
His longevity in the game is something that South Africa tried to make a weakness. In the BBC’s Rugby Union Podcast, former teammate and now broadcaster Tommy Bowe compared Sexton to Lazarus in his performance.
He explained: "Pieter-Steph du Toit put so many shots in which was marginally late and he kept getting back up.”
Cramping up late on, hardly surprising, Sexton was replaced by Jack Crowley in the 75th minute and was given a standing ovation in the French capital by his adoring fans.
"There were so many times he was down on one knee and you thought he's going to come off, but he is like Lazarus - he just keeps coming back," said Bowe.
Now the question remains, can Sexton take this team all the way?
Firstly, Ireland need to break the quarter-final curse. They have never made it past that point in any previous World Cup, but, perhaps they have never looked better.
Ranked number one in the world, with a coach in Andy Farrell who has his gameplay nailed down, they have proven they can match it with the best by beating the reigning champions in South Africa.
The biggest issue with Johnny Sexton will be his fitness. The Rugby World Cup is a notoriously attritional tournament and to make it to the end, Ireland will play four games in five weeks and look likely to face New Zealand in the last eight if Pool A and B follow the form book.
That is a test for anyone, let alone someone two years away from his 40th birthday. However, this works both ways. The experience of seeing it all will be a huge benefit to the Irish side and they will circle around their leader to make sure he can make it through - the other teams’ task will be to get to him early with hard-hitting tackles.
But, given how determined Sexton seems to be at this World Cup, he will not leave this field under his own steam if he doesn’t have to.
Ireland seem on the brink of history and have shown already that they are a force to be reckoned with. Now, with four huge games left to play, Sexton will have to continue producing performances akin to Lazarus if he is to keep the music playing.
