It was hardly vintage stuff from Les Bleus in a brisk Finnish capital, but the Arsenal midfielder's neat strike from a Karim Benzema assist in the first half allowed Didier Deschamps' first competitive game in charge to end in victory. France also had Hugo Lloris to thank, though, for a crucial late save as the hosts pushed forward in search of an equaliser.
In a five-team qualifying Group I that features reigning World and European champions Spain as the heavy favourites, France had little option other than to win against a Finland side ranked a lowly 96th in the world.
Deschamps is making his stamp on the team early on, and his line-up here included just five survivors from Laurent Blanc's final match in charge, the 2-0 defeat to Spain in the quarter-finals of UEFA EURO 2012 in June.
There were also four changes from the side that started against Uruguay in Deschamps' first match in charge last month, with Anthony Reveillere, Yohan Cabaye and the suspension-free Jeremy Menez brought in, alongside Diaby, who was making his first appearance for France since June last year.
Yet the visitors, with several players sporting gloves in the autumnal chill of Helsinki, got off to a rather shaky start. Winning just his second cap in central defence, Mapou Yanga-Mbiwa had to make a vital last-ditch intervention to prevent the lively Kasper Hamalainen from pulling the trigger in front of goalkeeper Hugo Lloris, and Cabaye was booked for bringing down the Finland attacker shortly after.
However, France produced a moment of class to take the lead in the 20th minute. In something of a role reversal, Karim Benzema did the spadework in midfield before slotting the ball through for Diaby, who got in between Finland's two centre-backs and finished neatly through the legs of goalkeeper Lukas Hradecky. After a year ruined by injury, it was the 26-year-old's first goal at international level.
Hradecky then saved a Benzema shot from a tight angle, while the French defence comfortably dealt with whatever the home attack could throw at them.
The second period was somewhat more tense for the visitors, however. After Patrice Evra had given away possession in midfield, Finland flowed forward, and only a block from Anthony Reveillere stopped the Schalke 04 striker Teemu Pukki from equalising.
And Deschamps' then had Lloris to thank for coming to the rescue in the closing minutes. The France captain got down at his near post to keep out a goal-bound header from the unmarked Tim Sparv with six minutes remaining.
As well as making sure of the win for Les Bleus, Lloris might also hope that such a save can convince his new manager at Tottenham Hotspur, Andre Villas-Boas, to make him the club's first-choice keeper in the Premier League following his recent move from Lyon.