Veteran French striker Jerome Vareille grabbed the winner five minutes from time for Airdrie United.
St Johnstone, many people's title favourites, dominated the first half, Mark McGeown producing a fine save to stop Paul Sheerin's goalbound header.
Airdrie rallied after the break and should have gone into the lead when Scott Wilson headed wide a cross from Mark Roberts.
But it was the Second Division champions who secured the victory.
Airdrie Utd: McGeown, William Wilson, Lovering, McGowan (Scott Wilson 18), McManus, Dunn, Vareille (Docherty 87), Marvyn Wilson, Coyle, Gow (McKeown 87), Roberts.
Subs Not Used: Barkey, Hollis.
Booked: Marvyn Wilson.
Goal: Vareille 85.
St Johnstone: Cuthbert, Tait, Malone, Hannah, Rutkiewicz, Maxwell, Stevenson (Hay 69), Bernard, Hardy, Moore, Sheerin (Marshall 87).
Subs Not Used: Nelson, Fraser, Baxter.
Referee: H Dallas