Joe Murphy's filly had to shoulder a 3lb penalty for winning a Listed prize at the Curragh at the start of the month but was a well-supported 9-4 favourite to follow up in the hands of Gary Carroll.
She broke smartly from her high draw and soon moved across the track to take up the running.
Rehana, winner of the Athasi Stakes at this venue in May before disappointing in the Irish 1,000 Guineas, did her best to bridge the gap, but Only Mine galloped on relentlessly for a one-length victory.
Music Box finished well to beat Rehana to the runner-up spot by a head.
Murphy said: "She was always travelling well and I think she's improving all the time.
"She'll probably go for the Group Three at the Curragh on August 13 (Phoenix Sprint Stakes) and then we'll think about the Group One at Haydock that she was seventh in last year (Sprint Cup, September 9).
"Anything bar extremes in the ground is fine for her.
"She is very genuine and very consistent and I love her to bits. She never runs a bad race and is building up a fine pedigree page."