In McClaren's first competitive fixture at the helm, Newcastle came from behind to lead at St James' Park after Graziano Pelle had opened the scoring midway through the first half.
Papiss Cisse's close-range finish ensured the two sides went into the break level, before debutant Georginio Wijnaldum - a close-season arrival from Dutch champions PSV - capped his impressive bow with an equally fine header at the start of the second period.
But Southampton, led by Ronald Koeman's brother Erwin as the manager recovers from Achilles surgery, showed their battling qualities and levelled with 11 minutes remaining when Long headed in Dusan Tadic's cross.
It was an impressive response from the visitors, who started with eight of the XI in action just three days previously against Vitesse in the UEFA Europa League.
For Newcastle, it was an encouraging start to life under McClaren, having narrowly avoided relegation at the end of last season.
