Essex scored 15 runs off the final over from Dirk Nannes to see off South Group rivals Surrey on an emotional night at Chelmsford.
Playing for the first time since the death of Tom Maynard in the early hours of Monday morning, the visitors looked to be on their way to victory until Tim Phillips produced some late heroics.
The left-arm spinner followed up his three-wicket haul by blasting Australian Nannes for a six and a four before getting a single off the very last ball to see the Eagles to glory.
James Franklin was at the other end, the opener carrying his bat to make an unbeaten 63 from 51 deliveries with two fours and three sixes.
The New Zealand all-rounder shared in a fourth-wicket stand worth 57 with James Foster (23) after Essex had slipped to 36-3 at the start of their reply.
The balance of power seemed to shift back in Surrey's favour when they reduced their opponents to 102-6 after 14 overs, only for Franklin and Phillips to deny them in a thrilling finish.
Surrey, who were missing captain Rory Hamilton-Brown on compassionate grounds because of the death of his great friend Maynard, had earlier managed to make 144-9 from their 20 overs.
They started brightly enough with opening pairing Steven Davies and Jason Roy taking 38 from the first four overs.
But once they were separated wickets fell steadily, Reece Topley (3-28) doing the majority of the damage. He received good support from Ryan ten Doeschate, who collected 2-20 from his four overs.
Roy top-scored with 36 from 20 balls, all but four of his runs arriving in boundaries, while Matthew Spriegel made 35 from No.6 in the order.
The problem for Surrey was that they managed just 28 from the final five overs and their failure to add impetus in that period was to prove their downfall.
Essex donated 50 pence from each of their matchday programme sales which resulted in them being able to give £320 to the charity of choice of the Maynard family.