That means the remarkable Oamaru man will be 105 when the longstanding arrangement is reviewed by his nephew, Don Hay.
Mr Wilson, who turns 100 today, regularly mows Mr Hay's lawns, using a ride-on lawn mower, with great skill.
"He's a very fussy man, this joker, in everything he does. I like to make a good job, myself. I'm more interested in doing a good job than leaving a little line of grass behind," Mr Wilson said.
And he always got "excellent" reports from blood tests - "so I might cut his blinking lawns after 100", he said with a chuckle.
Still fiercely independent and with a quick wit, Mr Wilson lives alone, cooks for himself and tends a vegetable garden.
He painted an 80m picket fence in his late 80s, made a tomato house in his 90s, was catching lambs at tailing until he was 95 and drove a car until he was 96.
He now rides a mobility scooter and, while Mr Hay picks him up for his lawn-mowing duties, he did once decide to take the back roads on his scooter from his north Oamaru home to Mr Hay's property on T. Y. Duncan Rd.
He conceded it was a bit too far as he "nearly ran out of juice".
Born in Christchurch, Mr Wilson was the first hired cowboy on Longslip Station, near Omarama, starting work in 1926 aged 17.
He recalled the hard, cold winters, icicles on his woollen blankets from his breath, along with trying to defrost boots at night.
At night, he would put a river stone in the fire and use it as a hot-water bottle in an old sock.
"With the hardest of the black frosts, someone used to ride around the ewe blocks looking at only the sheltered dark gullies for ewes frozen to the ground - the weaker ones used to get caught and died.
"Sometimes, in the hard winters, the boss, mate and I would be on the go looking at all blocks and the hell it was hard work," he said.
He spent nine days on the road driving sheep from Longslip to Duntroon and once took a mob of sheep through the main street of Kurow.
"They all went crook about a wee bit of s... going into the shop. It was not a very big mob, only about 500-odd."
Asked about his longevity, Mr Wilson said his late sister Celia Hay "put some sense" into him when he was young.
"She said, you must eat good, plain food . . . and keep that thing on top [pointing to his head] . . . in working order or the rest of the body will follow suit," he said.
Mr Wilson said it was important to stay active.
"Get out and do something even if you don't do it and just walk around the block.
"Keep thinking of something else to do. You can't sit in the chair all the time .... you're only ruining your body. I try and get out as much as I can."
Mr Wilson and his late wife Ruth had eight children, of which six survive.
He is looking forward to celebrating his birthday this weekend, saying it will be "superb" to catch up with family.
