The Irish province, runners-up in this competition last year, extended the only unbeaten record left in top-flight European rugby with a 25-6 bonus victory at Franklin's Gardens.
Tries from the outstanding Andrew Trimble and fellow wing Tommy Bowe had put Ulster, runners-up in this competition last year, in command at the interval.
Ulster pushed on to seal their 13th consecutive victory of the campaign with a slick try from full-back Jared Payne and Dan Tuohy's last-minute try, which sealed the bonus point.
The Saints have now suffered back-to-back European defeats and they must win at Ravenhill next week to stand any chance of keeping their Heineken Cup ambitions alive.
"That was not good enough from us," said Hartley, the Northampton captain.
"We speak about intensity, they brought bags of it and we didn't bring enough. They are unbeaten and they showed why. They are a good team but we made them look better than it could have been.
"We had internationals returning, injured players returning and our strongest team on the field but you can't just turn up and expect it to happen and I think that happened tonight. We got showed up by a good team.
"They did exactly what we would like to do to another team. They forced us into errors. It is not good enough."
Northampton have struggled to make their mark in Europe since finishing runners-up in 2010 but Ulster have not suffered the same problems.
Ulster have won 13 consecutive matches this campaign. They have 14 points from a possible 15 in Pool Four and look every inch leading Heineken Cup contenders again.
"That was a bit of a statement," said Ulster coach Mark Anscombe.
"We weren't even thinking about the bonus point. We were more than happy to come here and take the win.
"We are not getting carried away. Northampton have too many quality players and they will bounce back from this performance.
"We have to go out and do it again next week or this will count for nothing."
Ulster fear their captain and second row Johan Muller may have suffered a broken arm as he defended his line against a late Northampton attack.
If Ulster are without Muller, Northampton may be sweating on the availability of Hartley who could be in hot water with the disciplinary officials after an off-the-ball clash with Rory Best.
"I don't think it was an incident," Hartley said.
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