But she saw her lead cut to just one shot after 10 holes, only to respond by birdieing the next three.
The former world number one then started her final round with a triple bogey seven, but Webb dropped four shots in the first three holes to effectively end her challenge.
With Shin now pulling away from the field, a torrential downpour looked like it might send the tournament to a Monday finish.
However, the delay in play was for only 10 minutes and although she bogeyed the 17th, by that time Shin was already out of sight.
Park birdied the par-five 18th to take second, one shot ahead of America’s Paula Creamer, with Japan’s Mika Miyazato a shot further back.
Scotland’s Catriona Matthew, a winner at Royal Lytham three years ago, sneaked into a tie for 10th on seven over, while world number one Yani Tseng, aiming for a third successive British Open, finished on 11 over.
Fifteen-year-old Lydia Ko – born in Korea but now representing New Zealand – was leading amateur at nine over, two better than England’s Holly Clyburn, a 21-year-old from Lincolnshire.