Both are active members of the Okaihau Lions Club and encourage young farmers through the Farm Cadet Scheme and by hosting agricultural exchange students.
Since 2006 Mr MacMillan has been chairman of the Northland Regional Animal Health Committee, while Mrs MacMillan is the organisation's secretary and minute-taker. Right now they are organising a Vietnam vets' reunion at Waitangi in June.
A full list of the causes and organisations they have been involved with would fill up much of this page.
Mr MacMillan was raised in Broadwood, North Hokianga, meeting his English-born wife-to-be via the Waikato Tramping Club when both were living in Hamilton.
They married soon afterwards, starting their farming career as sharemilkers at Waihi before returning to the North.
They have three children, twins Nancy and Jenny, 32, and David, 29.
Mr MacMillan said his community work began when he voiced a strong opinion at a Federated Farmers' meeting, prompting the dairy section chairman to say, "I'll find you a job". And the jobs have kept coming ever since, including as the organisation's Okaihau and Bay of Islands president and later as a Northland Dairy Company director. The 1980s, when interest rates went through the roof, were an especially busy time with many farmers in need of help.
"Most of the jobs are rewarding, a lot of the time you can help people without having to do an awful lot," Mr MacMillan said.
He landed the job of organising Okaihau's Anzac Day service 20 years ago when numbers were dwindling and some of the old soldiers wanted to give it up: "I said I'd try it and see how it goes, and 70 people turned up."
Mrs MacMillan said she had been influenced by the uncle and aunt who brought her up in England.
"They did a huge amount of work for other people without reward. I really enjoy it. You meet people with huge generosity of spirit, you have fun, and why not?" she said.
One thing the pair was certain of: "We couldn't have done any of this without support from each other."