The 17-year-old, who was competing in the women’s 16-19 grouping, completed the 750m swim, 20km bike ride and 5km run in 1 hours, 12 minutes and 41 seconds.
The triumph placed her 12th in her age group and 61st overall.
Rosie said: “The race was the toughest thing I’ve ever done! My swim went well but it was a tough bike course with strong head wind (it is the Windy City after all!)
“My run was extremely hard … it was 30 degrees and seeing as I’ve only ever trained in England I wasn’t used to racing in the heat. Many competitors in front of me collapsed due to the heat, so I backed off the pace slightly so I wouldn’t do the same.
“I was extremely happy to win first Brit home. The atmosphere was amazing and I have enjoyed every minute of it … even if it has been painful! The venue was beautiful, everyone was really friendly and the experience is one I will never forget!”
Nearly 2,000 age group athletes, aged between 16 and 19 and representing more than 40 nations, pushed themselves to the limits in search of ITU Age Group Sprint World Championship titles on Thursday (17 September).
Set in iconic downtown Chicago, the course saw athletes swimming in Monroe Harbor on the shore of Lake Michigan and racing the Windy City’s streets, twisting around historic Buckingham Fountain with views of Chicago’s skyline.