The City of Ely bowler edged out David Hubbard, from the Adur club, in Southwick, 21-18 to add to his previous successes in 2002 and 2011.
Hubbard, the champion six years ago, became a losing finalist for the second year running.
Spectators were royally entertained as Harlow, a long-time member of the World Bowls Tour top 16, was outbowled for much of the game, but showed that determination is as important as skill as he fought his way back into contention.
Having scored first, a single on the opening end, Harlow found himself 9-3 adrift after six ends, and 15-9 in arrears after 12.
Trailing 18-14 he faced a match lie on the 19th end, but took the jack through for a count of two, and finally moved in front on the 21st end.
“I’m still trying to work out how I did it,” Harlow said. “David was playing so well, and I just dug in there when things were looking bleak - and it all came out right in the end.”
From 18-14 down after 18 ends, 46-year-old Harlow got home with an unanswered streak of 2-1-3-1, and returned a 21-18 card that did scant justice to the way 28-year-old Hubbard had played.
n In the women’s singles final, Ipswich ace Katherine Rednall became the youngest player to win the national women’s singles title when she overwhelmed Lincoln’s Penny Strong, 21-6, in a one-sided final.
“That was the one I wanted to win,” the 19-year-old said. “I’ve now won the England junior singles and Champion of Champions titles, and, of course, I won the WBT women’s world indoor singles title at Potters two years ago.
“But this is the hardest of all to win, and was top of my wish-list.
“On Thursday, I really thought I was going out in the first round. I was staring defeat in the face when I was 20-17 down against Jo Morris, but I managed to pick up a cheap four to win, 21-20!”
Rednall, who skipped her mum Christine and friend Anna Chalk to the triples title on Wednesday, will now represent England in the two events, singles and triples, at the British Isles Championships in Wales next March.
A disappointed Strong, who was runner-up in this event 34 years ago, said: “I just didn’t turn up. The carpet seemed to be a different pace from yesterday, and I just couldn’t find the length.
“But Katherine played brilliantly and fully deserved to win.”