The 32-year-old, from Sheffield, was in control from the start of the match at the National Squash Centre in Manchester, going on to triumph 11-9 11-3 11-3 in 49 minutes – his third victory over Willstrop in the final.
Willstrop, from Leeds, struggled to get going throughout and may have been feeling the after-effects of his gruelling 99-minute semi-final against Peter Barker on Saturday when he came from 2-0 down to reach the final.
Matthew, who had previously won the title in 2006, 2009, 2010 and 2012, had generally had a more comfortable week, although he was pushed hard by Tom Richards in their last four clash, staying on court for more than an hour before booking his place for the final for a sixth time.
“It feels fantastic,” said Matthew afterwards. “People have made a big thing about me winning five titles but, to be honest, I don’t count them, I just want to win them.
“James is a great competitor, and while he won’t make excuses himself, I can do it for him because he was clearly not at his best today and I have the utmost respect for him.
“There is a big rivalry between us, the same as there is with Ramy (Ashour, world No 1) or Greg (Gaultier) or any of the top players.
“But, if that rivalry keeps putting bottoms on seats – as it has done here this week – long may it continue. The atmosphere for the last two finals has been sensational.
“This deserves to be an Olympic sport.”
Willstrop, as ever, was gracious in defeat, accepting he had been beaten by the better player on the day.
“All credit to Nick – he put me under more presure physically than anyone has for a long time,” said Willstrop, who was himself gunning for a third title.
“I don’t know if it’s the accumulation of matches or I’m just not good enough physically at the moment to deal with what he always throws at you.
“But he fully deserves his fifth title.”
The next event for the Yorkshire duo will see them head to Richmond, Virginia, for the North American Open, an event won by 29- last year.
In the women’s draw, Alison Waters collected her third title by defeating reigning champion Laura Massaro 11-7 7-11 12-10 6-11 13-11.
“It was tough all the way, probably one of the best games of squash I have ever played in,” said Waters, winner of the title in 2008 and 2010 and who had ended the hopes of Harrogate’s Jenny Duncalf in the semi-finals on Saturday.
Duncalf, 30, was aiming for a third title of her own and had shown good form all week, but went down to her England team-mate 11-5, 11-9, 11-8 in 43 minutes.