Mr Smith was "very proud, humbled and honoured" to have the role of mayor.
He said a high voter turnout in the byelection - 42.5 per cent - was a wonderful thing for civic pride in Kaipara district.
Mr Smith was looking forward to working with everyone "with my ear to the ground and my shoulder to the wheel".
His work started immediately, as he headed from the swearing-in straight to Whangarei to meet with the region's other mayors as part of the mayoral forum.
Next week, the consultation document for Kaipara's Long Term Plan will be finalised.
"We're moving fast - but I'm very honoured and humbled to be here today and looking forward greatly to serving the people of the Kaipara District."
That sentence had the beef and sheep farmer choking up again.
After the official ceremony had wrapped up, Mr Smith told the Northern Advocate it was very special.
"I feel the weight of the generations before me," he said of the emotion he showed.
His parents, Clive and Colleen Smith, were among those in the crowd.
"We're incredibly proud. We know Jason's heart is very genuine. He's always been 100 per cent in everything he does," his mother said.
His father said his son is very passionate about the Kaipara district.
His mother said she too may have shed a tear.
"It's a good team he'll be working with."
The byelection was held after former mayor Greg Gent resigned last year.
