Durham 299 for 6 (Clark 92, Steel 77, Collingwood 73*) beat Nottinghamshire 297 (Hales 104, Root 66, Read 61, Wood 3-62, Weighell 3-66) by four wickets
Paul Collingwood rolled back the years to help Durham complete a remarkable run chase under the Trent Bridge floodlights.
The 40-year old scored an unbeaten 73 not out as his side completed a four-wicket victory, with five balls remaining, having been set to score 298.
Collingwood scored his runs from only 47 balls, hitting a six and eight fours, the last of which sealed a highly-impressive win.
Clark made 92 from 94 balls and Steel accompanied him with 77 from 109 balls but when they fell it was left to Collingwood to guide their side over the line.
"When you put in a batting performance like that as team it is very satisfying to see it through right until the end," Collingwood said. "We were in a bit of trouble but the way that Steely and Clark stuck at it, they kept us in the game all the way
"It was fantastic to see that and they gave us a chance at the back end. Today was my day. I struggled when I first went in with the pace of the pitch but thankfully I got a couple out of the middle of the bat. We're delighted with the win"
Earlier, a century from Alex Hales had been the backbone behind the Nottinghamshire's innings. The England opener scored 104 and with half centuries from Billy Root and Chris Read, Notts reached 297 before being bowled out one delivery short of their 50-over allocation.
During the Notts innings Mark Wood and James Weighell each picked up three wickets for the visitors but Hales was dropped on three separate occasions and made Durham pay by advancing to the 15th one-day hundred of his career.
Hales scored his runs from 103 balls, with eight fours and three huge sixes, one sailing straight through the open windows of the radio commentary boxes on the third floor of the Radcliffe Road Stand.
Weighell put Hales down on 27 and 93, both from the bowling of Chris Rushworth and also had the misfortune to see wicketkeeper Stuart Poynter grass a low edge when the batsman was on 43.
Rushworth had put the hosts on the back foot early on, by removing Michael Lumb in the first over and Riki Wessels in the fifth but Samit Patel, in his benefit match, played nicely before lifting to the point boundary for 28.
Hales, who passed 5,000 one-day runs during his innings, was eventually dismissed by Weighell, with Ryan Pringle taking a well-judged catch in the deep.
Root and Read combined with a well-paced sixth wicket stand of 101 in 11.5 overs, before both fell to consecutive deliveries from Paul Coughlin. Read made 61 from just 38 balls, with seven fours and two sixes, with Root falling on 66 from 86 balls, with six boundaries.
That heralded a collapse which saw Notts slide from 288 for five to 297 all out with Wood taking all of his wickets in the final over to finish with 3 for 62. Weighell, the competition's leading wicket-taker, took his tally to 16 with 3 for 66.
Notts would have been desperately keen to dismiss Jennings early on but couldn't have envisaged getting his wicket as early as they did.
The left-hander, with two centuries already in this year's competition, clipped his first ball straight into the hands of Patel at point. Bowler Ball then struck again as Michael Richardson edged to slip to leave the chase in tatters at 8 for 2.
Clark and Steel both passed their previous best scores in a stand that spanned 32 overs, although Notts missed a couple of chances to end their stay.
Mullaney put down a difficult slip catch when Steel was on two and then Hales dropped a far easier effort on the deep midwicket fence when the same player had 41.
Pattinson, playing his 50th one-day match, trapped Clark lbw and then caught Steel at the start of the next over, off Gurney.
Entering the final 10 overs Durham needed 93 runs from 60 balls but were powered to their target with Coughlin and Stuart Poynter both hitting sixes, before leaving it to Collingwood to hit Gurney for the winning boundary.
The victory lifts Durham up to second place in the north group, leap-frogging both Notts and Worcestershire and despite their two-point penalty at the start of the campaign they are now very much on course to qualify for the knock-out phase.