Sven Struver finished alone in second at 5-under-par 283 while Bradley Dredge and Malcolm Mackenzie were one shot further back at 4-under-par 284.
Tunnicliff began the day tied for first with Struver and David Gilford. While Struver and Gilford struggled early, Tunnicliff's game was sharp as he lifted himself atop the leaderboard at De Vere Slaley Hall Golf Club.
Tunnicliff bogeyed the opening hole but recovered with a 30-foot birdie at the par-4 third. He moved to 7-under with a chip-in birdie from just off the green at the fifth and soon had a three-shot edge over his challengers.
The 33-year-old increased his lead to four after another remarkable chip-in birdie at the par-4 eighth but ran into trouble with a bogey at the very next hole.
Tunnicliff recovered with birdie at the 11th and drained an 18-foot putt for birdie at the 15th to reach nine-under.
The Englishman was brilliant with the putter as he saved par from the fringe at the 16th. On the closing hole, Tunnicliff ran home another putt from the fringe to save par and earn the emotional, breakthrough title.
Only two weeks ago Tunnicliff's mother passed away after a bout with cancer. She had one request for her son: a tournament win.
'Two days before she died she told me to go out and win a tournament so I had to get out here and try to do just that,' said Tunnicliff.
Tunnicliff has battled hard throughout his career after turning pro in 1989. He has struggled through the Challenge Tour and visited the Qualifying School a total of 12 times. Before Sunday's win, his best finish on tour was sixth at the OKI Pro Am in 1996.
'What happened to my mother made me dig in a little bit harder,' said Tunnicliff, who earned a two-year exemption on the European Tour. 'She gave me quite a bit of positive strength. She was positive right to the end, so I just took as much from that out on the golf course this week.'
Struver struggled on the front side with three bogeys before finally recording a birdie at the par-four 10th. He added another birdie at the very next hole but found trouble with a bogey at the 12th.
The German responded with a birdie at the 15th for his first top-10 finish of 2002.
Diego Borrego, Jean Hugo, Nicolas Vanhootegem and Brian Davis finished tied for fifth at two-under-par 286.
Full-Field Scores from The Great North Open