Leicester City and manager Brendan Rodgers fully merited their victory as they were much the better team in a low-key semi-final.
They were the more composed and ordered side in a game where chances were at a premium and it took Iheanacho, who scored the winner against Brighton in the fifth round and was on target twice in the 3-1 win against Manchester United in the quarter-final, to make the decisive contribution.
His goal secured a place in the final against Chelsea, who the Foxes beat in the Premier League in January.
And while they will be underdogs, Leicester have enough quality and resilience to be quietly confident.
This FA Cup final appearance is further evidence of the fine work being done by Rodgers, who also has Leicester right in the mix for the Champions League places next season.
Now they have the chance to turn all this endeavour into silverware at Wembley against Thomas Tuchel’s side.
Southampton come up short
Southampton will leave Wembley with bitter regrets as they failed to produce anything near their best and can have no complaints about the outcome.
They showed little of the intensity and energy that is their trademark when at their best under manager Ralph Hasenhuttl.
The Saints lacked ambition and positive intent apart from a brief revival a couple of minutes after Leicester scored, when Diallo was not far off target and there was a scramble after a corner.
Main marksman Danny Ings cut a frustrated figure, starved of service and reduced mainly to chasing lost causes.
This will be a real blow to Southampton, who fancied their chances in the FA Cup this season and had a big opportunity to reach the final, but they came up short and must now return to ensuring their Premier League status.
Fans make Wembley return
This was the biggest crowd at a football match in England since the country went into lockdown in March last year – and what a glorious sound it was as the Leicester and Southampton players came out to cheers from 4,000 fans inside Wembley, reciprocating the gesture as they lined up before kick-off.
It did not have the partisanship and passion of the tradition football crowd because this was a test and research event with only locals and key workers in attendance.
There were moments, however, especially when Leicester scored and when Southampton substitute Theo Walcott came on to a very loud, mixed reception – presumably from Arsenal fans inside Wembley who idolised him from his time with the Gunners and Tottenham supporters who took the opposite view.
There were resounding cheers when the supporters were thanked over the public address system for their attendance and the hope is that this test event will help navigate a route out of the behind-closed-doors occasions that have been the way of things during lockdown.
The sight and sound of fans added to the spectacle, even when most of those watching this FA Cup semi-final were neutrals.
‘We have the chance to create history’ – what they said
Leicester boss Brendan Rodgers, speaking to BBC Sport: “It feels amazing. I thought we deserved it, the players worked so hard. Commiserations to Southampton, they are a terrific team.
“We got the goal which was a great piece of play by Jamie [Vardy]. We had two or three moments after that to close it out. When you are 1-0 up, you have to show that self-control and stay calm and we did that.
“Even thought there were only 4,000 fans, it felt great. It will be amazing when we get supporters back into stadiums. I did hear some Leicester supporters in there and it was really nice.
“We have the chance to create history. That is what this game is about, creating a memory. I have been made aware since I’ve been at Leicester how important this cup is for the supporters.
“When we arrived here, that was the ambition. We said we wanted to be competitive. From a football perspective we wanted to be able to compete and we have been able to do that.
“Now we have a trophy to genuinely go for. When you fail it is an integral part of being successful. We missed out last year, so a great credit to the players this time.”
On Kelechi Iheanacho: “His confidence levels are very high. Some of his set-up play and build-up play was a bit loose today but his confidence was not affected by that. He and Jamie [Vardy] are a real threat but it is very much a team effort.
Southampton boss Ralph Hasenhuttl, speaking to BBC Sport: “We conceded our first goal in the FA Cup. [It is] Hard to take because we haven’t been the worst team. We tried to put pressure on them. We saw a not very good football game to be honest, with both teams a little nervous.
“That such a goal decides such a game is a pity for us. We had a good run and we wanted to get to the final to give our fans the chance of another final. In the end especially in the final third we didn’t have good moments. We couldn’t get a shot on goal and this is not enough.
“We had the chance to get to the final. Try again next year.”
Leicester’s long wait ends – the stats
- Leicester City have reached their first FA Cup final since 1969, a gap of 52 years, the longest between finals since Portsmouth went 69 and Cardiff went 81 between finals before facing one another in 2008.
- Southampton have now been eliminated in eight of their past 10 FA Cup semi-final appearances, two of which have come in the past four seasons (2017-18 vs Chelsea, 2020-21 vs Leicester).
- This was only Leicester’s fourth win in 15 appearances at Wembley (D1 L10) and their first since the 2000 League Cup final against Tranmere, while Southampton have lost eight of their 10 matches at Wembley (W2), losing their last five in a row.
- Since his debut in the FA Cup in January 2016, Kelechi Iheanacho has scored more goals in the competition (first round onwards) than any other player, netting 14 times in 19 games.
- Iheanacho is the first Nigerian player to score 15 goals in all competitions in a season for a Premier League club since Odion Ighalo in 2015-16 for Watford, who scored 17.
- Southampton failed to have a single shot in the first half, the first time that had happened since November 2019 against Everton in a Premier League game.
- Only Sheffield United (13) have lost more games in all competitions in 2021 among Premier League clubs than Southampton (12).
- Jamie Vardy has gone 11 games without a goal in all competitions for Leicester, his longest goal drought since a run of 16 games between September – December 2016.
Leicester return to action when they host West Bromwich Albion in the Premier League on Thursday, 22 April (20:00 BST).
Southampton travel to Tottenham for their next match in the Premier League on Wednesday, 21 April (18:00 BST).