McCarthy opened with a first-round 66 at Hillside and was heading the pursuit for one of the three places on offer from the final qualifying event in Lancashire with just two holes to play.
When he pushed his second shot wide at the par-5 17th he thought the crowd would find the ball for him, but it was lost.
The 29-year-old kept a cool head by shedding only a shot – his first of the day – but admits a bit of temper kicked in on the final tee.
“I was a little bit angry, but it kind of helped me,” said McCarthy, who last week rose to ninth in the HotelPlanner.com EuroPro Tour order of merit after closing with a 63 to finish third in the Grenke Championship at Cumberwell Park in Wiltshire.
“There was a good crowd walking round so I thought I’d find my second shot at 17, but I ended up losing it and made a good bogey with my second ball. On the 18th tee it fired me up a little bit and I took four-iron and ended with 187 yards to go – and then hit probably my best shot of the day.
“It was downwind and out of the semi-rough and I took nine-iron and put it to about four feet and holed for birdie.”
The final-hole birdie left him on seven under par after adding a one-under 70 to his 66, which ultimately gave him second place behind Delamere Forest’s Haydn McCullen.
Chorlton-Cum-Hardy’s Adam Hodkinson, a shot back of McCarthy, took the third Open spot available at Hillside.
“I’m a little bit in shock, I think,” said McCarthy of his achievement. “It’s your dream as a kid to get through and play in the Open and to achieve it is pretty awesome.”
He had gone close in final qualifying at Gullane previously, placing fifth, but in other attempts at Hillside had not gone close despite rating it as one of his favourite courses.
Undoubtedly his form on the EuroPro Tour this season has buoyed his confidence, two third-place finishes and a 10th-place showing in five events underscoring his consistency.
It let him tee off at Hillside with the mindset of not just claiming an Open berth, but winning the 36-hole final qualifying event.
Coming up just short was initially a disappointment, he conceded – but only initially.
“When I was out there I was playing for top spot, playing to win the event, but after I’ve come in and known that I’ve qualified that pales into insignificance,” he said.
“My thinking was that there was a tournament to be won, a tournament to play for, and despite falling short I still qualified.
“I think if you play to qualify you might end up coming up short.”
The first Open McCarthy can recall is etched vividly in his memory – because he was present as a seven-year-old when America’s John Daly won at St Andrews.
“I was there in the stands,” said McCarthy, who laughed and added: “I especially remember there was a streaker.”
McCarthy is on a hot streak himself, with his eight-birdie 63 in Wiltshire last Friday not even qualifying as his lowest round of the year.
That was a 62 at Warrington in April, which gave him victory on the day in the 1836 Tour event.
Today he will tee it up in Northumberland hoping to add further gloss to his achievement of earning a spot in the Open by winning the Dawson & Sanderson Classic, the latest EuroPro Tour event.
“All I can do is keep putting myself in those positions [high on the leaderboard] and hopefully at some point a win will come from it,” said McCarthy.
Who right now would bet against him doing that?