Trailing 0-2 from the morning foursomes, captain Tegwen Matthews' team of five - Kelly Tidy, Amy Boulden, Holly Clyburn, MacDonald and Pretswell - staged a remarkable recovery in the singles to win the match 4-3.
A Pretswell win out in the country was cancelled out by a defeat for MacDonald, leaving GB&I 1-3 in arrears with three matches - all of which could go either way - to finish.
Remarkably, against the odds, Clyburn, Boulden and Tidy won the last three singles all on the last green as excitement mounted among home fans.
GB&I were able to preserve their 100 per cent record over the week, having beaten all four of the opposing countries.
New Zealand, whose last game was their 4-3 defeat by GB&I on Saturday, finished runners-up with three wins out of four.
Matthews said: "I'm simply over the moon with delight. It was an absolutely amazing team win to finish with, under terrific pressure after Canada won both the morning foursomes.
"But the GB&I players dug so deep to come up with what was needed - and to win the last three matches on the 18th green was so exciting."
Pretswell put GB&I's first point of the day on the scoreboard by beating Rebecca Lee Bentham 6 and 5.
Pretswell birdied the third, fifth and seventh on her way to a four-hole lead at the turn. The Scot went five up with another birdie at the 11th and six up with a par at the 13th, which ended the match.
Christine Wong made it 3-1 for Canada by beating Kelsey MacDonald by one hole.
MacDonald had been two up at the turn with the help of birdies at the third and seventh. Wong won the 11th with a birdie 4 and squared the match at the next on a concession.
The Canadian had the momentum and she went one up for the first time in the match by winning the 14th with a birdie.
MacDonald was able to square it at the next with a par but Wong birdied the 16th to go one up again.
In a ding-dong finish, MacDonald levelled it at the short 17th but hit bunker trouble to lose the 18th and the match to the Canadian.
Clyburn kept GB&I's hopes of preserving their 100 per cent record by birdieing the last - holing a 15ft putt - to beat Anne Catherine Tanguay by one hole.
Boulden made it 3-3 overall by beating Nicole Vandermade by one hole in a pulsating finish.
Kelly Tidy delivered the winning fourth point for GB&I, beating Jessica Wallace by one hole. The last four holes were halved.
All members of the GB and I team now move on to Royal Porthcawl for the Vagliano Trophy match against the Continent of Europe on Friday and Saturday.