Essex survived a late wobble with the bat to successfully chase down Durham's 209-5 with four wickets and six balls to spare at Chester-le-Street.
Having been hammered by Hampshire in their opening match in this season's Yorkshire Bank 40 competition on Friday, the Eagles looked to be coasting to a morale-boosting victory when they reached 103 without loss.
Opening duo Rob Quiney, who made an even 50, and Tom Westley put them right on course with a century stand before the former fell to Scott Borthwick's leg spin.
Ravi Bopara managed only 10 and although Westley perished for a well-made 71 that included 10 boundaries, the visitors still remained on track to register their first points in Group B.
However, Mark Pettini (21) and James Foster (14) both departed in quick succession and when Graham Napier was caught at mid-off, Durham sensed a dramatic win could be on the cards.
In the end, though, Greg Smith and Tim Phillips kept their cool to see Essex over the line, the latter ending the game with a lovely lofted drive for four off Borthwick's bowling.
Durham will perhaps look back on a slow start with the bat that proved costly in the final outcome, the hosts at one stage having slipped to 39-3 after being put in by visiting skipper Foster.
Ben Stokes picked up the tempo with 51 before a throw from Sajid Mahmood caught him short of his ground; the all-rounder had hit a six and five fours in his 64-ball knock.
Gordon Muchall (49 not out) and Gareth Brees, who finished up unbeaten on 41, continued Stokes' good work with an unbroken sixth-wicket alliance worth 69 from 59 deliveries, though their final total always looked a little light.