And it was good way to cap their impressive Lotto Federation League season in which they improved from 2016 wooden spooners to third.
"It's great reward for them," King said.
"They're the ones that toiled out there on the pitch every week and pushed themselves to do more and more."
But City, the Horizons Premiership runners-up, did enough to show their Federation League credentials.
Fans can look forward to more match ups between the two next year with City coach Kelvin Francis confirming his side's intention to move up.
"I think we go into the [League] next year more than confident of being able to play and compete," he said.
"We've bought some good young players on."
After being delayed by a week due to wet weather, the match was played with the expected derby intensity and both sides had good chances to get on the board early.
Holden had a decent strike deflected onto the post by City keeper Glenn Graham in the opening minutes, while City's Cameron Hussey came flying onto a cross minutes later only to head over the bar.
But play settled after about 10 minutes and Athletic took control, hogging territory and possession as City struggled to play with any fluidity.
The pressure told in the end when Mailman finished off a sweeping counter attack to open the scoring in the 28th minute.
City chalked up a few yellow cards and that ill-discipline cost them just two minutes into the second half when giving away a penalty.
Holden stepped up to convert from the spot and Athletic were able to effectively shut out the game from there.
City did create at least two chances to pull goals back but lacked the finishing quality to match.
"It means a lot to the boys," King said.
"It's just a bit of silverware in the end but it means something. It's the big derby and it's bragging rights for the next season basically."
King was happy with how his side managed the game as planned.
"I was expecting a really good hard match - and it was - but I feel certainly in the first half we spent most of the time [in City territory].
"We controlled the game and we set the tempo. We got that early goal away which was really good and it's just being able to shut it out in the second half."
On the other hand, Francis was left frustrated at a game plan gone missing.
"We didn't play today - at all," he said.
"In patches, yep, but just not enough to put enough pressure on them to create enough opportunities.
"They played quite well. They recycled the ball well and we just didn't react to that."
Francis said the plan had been to play a short passing game and keep the ball at feet.
"And we just didn't do that. They didn't follow the instructions they were given so they've only got themselves to blame.
"We just didn't do what we talked about doing and paid the price for it."
Francis said being a lower league side was no excuse, in fact he felt City should have been a match for Athletic.
"They not a better team than us they just played better than us today."
Francis hoped the keep most of his squad together for next season's step up to Federation League.
Meanwhile, King said he would welcome a City move to join them.
"It would be nice to see that, because then we can have these derbies every few weeks," he said.
"That's always good for football in Whanganui."
In the Charity Plate competition, the Wanganui Athletic Weekenders bucked the odds-makers by winning both games to claim the silverware.
