The daughter of Reliable Man was sent out an $18 chance and was ridden patiently to instruction by apprentice jockey Chelsea Burdan.
Mistress Minx had been good in two trials, including a close-up fourth to highly-rated galloper Brando at Te Aroha.
"We had kept her reasonably settled in her trials and it was more just getting her there and educating her and keeping her as relaxed as possible," Tierney said.
"Earlier in the piece she had a tendency to get a bit hot, but we knew she had the ability there.
"She will definitely get over ground. In both her trials and her race yesterday she is quite happy to sit out the back and she relaxes so beautifully.
"She can take the pressure off herself and is able to wind into a race and she certainly showed a good turn of foot."
The feedback from Burdan was naturally positive.
"Chelsea has done a lot of trackwork with her and has done a great job," Tierney said.
"The main aim was to keep her out of trouble and it all went to plan."
The Matamata horsewoman, who oversees a boutique stable, suggested a long-range target like the Gr.1 New Zealand Oaks (2400m) would suit the talented filly.
"She is still a wee bit immature, so she may just need a bit more time to fill out," she said.
Meanwhile, stablemate On Demand finished a close-up third over 1200m in Rating 74 grade to advance her career record to two wins and two placings from five starts and looks on-track to land further spoils for her connections.
"I was happy with her run," Tierney said.
"She drew poorly and fought on well and we will step her up in distance next start.
"I have got about six in work, including a few young ones. I do prefer a smaller team and I have been involved in a few tricky horses that need that one on one attention and it definitely takes a lot of time to get them right.
"On Demand is one of those and she came with a few naughty habits but it is very rewarding when you see them start to fulfil their potential."