The race appeared to be Magical Lagoon's to lose after the withdrawal of long-time favourite Emily Upjohn.
However, the Ribblesdale Stakes winner was made to work until the dying moments for her first Group One success, with Aidan O'Brien's Toy pushing all the way to the finish.
She travelled smoothly and tracked outsider Show Of Stars who led the field in the early stages. Soon, the favourite loomed large and took over at the two-furlong pole.
Magical Lagoon showed all the stamina and battling qualities to first repel third-placed Cairde Go Deo and also win the duel with the runner-up.
"It's wonderful to win one Classic, to get two is absolutely amazing," said Harrington, who took the Irish 1,000 Guineas with Alpha Centauri in 2018.
"She's such a game filly, she only does as much as she has to do. She's just a lovely filly to train for Mr Zhang.
"I kept looking at what was behind her but I knew she'd keep staying, she stayed very well in Ascot. They probably didn't go as quick as they did in Ascot and she just ran a great race.
"Shane was in the right position and kept going forward. He rode a super race and she kept finding and finding.
"It's only her third run this year and I'd say she'll improve again. She's in the Yorkshire Oaks and after that I haven't really made a decision.
"I'd love to keep her in training next year. I hope so because Galileo's progeny improve and her brother was a much better four-year-old than three-year-old."
When asked how she felt after hearing of the withdrawal of Emily Upjohn on Friday due to travel difficulties, she added: "I was terrified because suddenly I'd become favourite.
"Before we were the underdogs and I love being the underdog and having a go at them, but it's worked out great."