The Duchy, coached by former Plymouth boss Graham Dawe, ran in seven tries to stun James Shanahan’s Hertfordshire team.
The visitors’ squad was built around National One club Old Albanian and National Two South champions Bishop’s Stortford, but the new-look Duchy team, with eight Redruth players in their starting XV, turned on the style in front of more than 3,500 people at the Recreation Ground.
The first 55 minutes were quite even. Cornwall had gone 19-7 up early in the first half but Hertfordshire pulled a try back before the break.
Both teams exchanged tries after the interval, with Hertfordshire converting theirs to move to within three points of Cornwall.
The home team, though, took advantage of a yellow card to the visitors to give themselves some breathing space before totally pulling away in the final 20 minutes, much to the delight of the large crowd.
Former Plymouth Albion flanker Sam Matavesi had another big game for the Duchy. He scored two tries, as did Redruth winger Lewis Vinnicombe.
Their other tries came from Tom Notman, Richard Brown and Camborne’s David Mankee, with Plymouth Albion full-back Matt Shepherd adding five conversions.
Shepherd was one of two Albion players in Cornwall’s starting line-up, with Robin Wedlake lining up at outside centre alongside former Plymouth skipper Jake Murphy.
Ex-Albion boss Shanahan started at fly-half for Hertfordshire and scored their first opening try, with Pierre Dotta, James Apperley, both of Bishop’s Stortford, and Old Albanian’s Sam Lunnon also touching down.
Cornwall, who have won the County Championship for the past two years, will face Lancashire in the final at Twickenham next Sunday. The teams have met in three of the last four finals, with Lancashire winning two of those meetings and Cornwall one.
Devon led 17-0 at half-time thanks to tries by Ivybridge players George Wilson and Ben Watts and seven points from the boot of Mitch Pinkus.
The home team were also 20-14 in front with 80 minutes up on the clock, but Kent scored two injury-time tries to claim victory and finish third in the six-team south pool.
Devon, in their first season back in the top flight, had been hit by a host of unavailabilities, including Cameron Setter, Tom Whelan, Lewis Paterson, James Harris, Tom Scoles, Sam Thomas, Matt Finn, Jack Fahy, Ashley Crouch, but they impressed first half against Kent.
Ivybridge half-backs Pinkus and Watts helped pull the strings as they built up a decent lead.
They took the lead in the 10th minute when Wilson went over for a try following great play by former Devonport Services number eight Martin Browne. Pinkus converted.
Devon should have increased their lead on 24 minutes but dropped the ball on the try line.
However, they did get a second try in the 35th minute when scrum-half Watts won the race to a kick forward to score under the posts.
Teenager Pinkus, making his first senior start, converted and also added a penalty to put Devon 17-0 ahead.
Kent, though, came out fired up for the second period and finally got on the scoresheet in the 50th minute when captain Tom Stradwick went over from close range for a converted try.
The Blackheath flanker added a second try in the 62nd minute from a catch-and-drive move, which Tunbridge fly-half Frank Reynolds converted.
Devon went 20-14 up four minutes later when Pinkus kicked a penalty under the posts after some good play by the home team.
But Pinkus was forced to go off after that due to an injury, which hit Devon.
However, it did look like they were going to hold out but six minutes into injury-time Kent forced their way over the try line courtesy of prop Aaron Cooper. Reynolds converted to put them 21-20 in front.
And in the dying seconds the visitors added another try, with Strandwick completing his hat-trick.