The Macnabs are well-known farmers in the Upokongaro and Makirikiri Valley area just north of the city on SH4.
The Cup has long been sought after ever since it was donated as a major prize in amateur riding ranks by Prince Henry, the Duke of Gloucester, who was the son of the reigning monarch King George V attended the Club's 1935 New Year's day race meeting at Marton.
It is on offer each year at one of the 15 Flair Series races and young Macnab made it her own when winning on Teddytwinkletoes at Rotorua last Sunday.
The result could not have been scripted better. Teddytwinkletoes is owned by her grandfather David Henry along with riding mentor Jo Rathbone. Rathbone, a former top jumps jockey is the right hand to trainer Kevin Myers.
Macnab has ridden three winners, two seconds, and a third to rack up 61 points in the Flair Series and with just one race to go at Te Rapa on September 25 and her nearest rival too far back to bridge the gap, the title will stay in Whanganui.
Macnab said her grandfather was one of the first to congratulate her and she has Myers, Rathbone and neighbour Simon Harrison to thank for her getting across the line.
"Simon dropped off a horse called Gingerbread Man last year and told me to get it ready to race - I've been hooked ever since," Macnab said yesterday.
"He's still my favourite horse, although I didn't win on him."
The majority of Macnab's amateur rides, including her three winners - Teddytwinkletoes, Viceroy and Selwyn - have been trained by Myers.
"Really I have Kevin Myers to thank for everything, although Jo and Simon have been great too."
Macnab is keen to forge a career in the professional ranks and weighing in at just 51kg fully clothed and her short stature makes her a prime candidate. That could be as soon as next year.
The teenager has already signalled she is unlikely to return to school in 2017.
Hard work and early mornings go hand in hand with becoming a jockey, but neither hold any fears for the young Macnab.
"I go down to Kevin's in Turakina every morning to ride trackwork before school," she said.