He said he told party leader Winston Peters that he was considering retiring from politics in the middle of last year.
"He expressed his wish for me to stay but ultimately understood the reasons for me considering a change in my career, and wished me and my family all the best for the future," Mitchell said.
Based in Tauranga, Mitchell has been at the centre of some questions around the NZ First Foundation, which is being investigated by the Serious Fraud Office.
Mitchell was said to be in charge of bringing in party donations, and used money from the foundation's fund for flights to Europe to "observe the EU elections" last year. His accommodation in the UK, was paid for by the right-wing Brexit Party.
The information was revealed in the annual Register of Pecuniary Interests – a list of MPs' assets and debts, as well as the gifts they have received and their overseas travel.
The list does not say how much money the items, assets or travel cost, but it does not appear on the list unless its market value is deemed to likely be above $500.
The Serious Fraud Office has said that it expected to conclude its investigation before the September 19 election.
The Electoral Commission has previously said the foundation received donations that should have been treated as party donations to New Zealand First.
"In the commission's view, the donations were not properly transmitted to the party and not disclosed as required by the Electoral Act 1993," the commission said in February.
Peters has always maintained that the foundation has not done anything illegal.
Last year Mitchell was removed from a bar after reportedly failing repeatedly to move out of the way of bar staff. He said he was escorted from the bar after a misunderstanding.
A bar he owned - the Mount Mellick Irish pub in Mt Maunganui - was also previously investigated by gambling authorities over concerns about underage gambling on pokie machines.
The pub ran a "leprechaun curling" competition, in which an oiled dwarf was thrown down a length of plastic, Stuff reported in 2018.
Mitchell received a suspended sentence after a fight with a gang member in 1997 who attacked him after being refused entry to a bar.
"I have been involved in hospitality for 25 years and, because you run bars and you stand on the front door, you do have, and I certainly have had, over the years, a lot of situations where you get put into very perilous situations," Mitchell told the Herald in 2015.
Mitchell, who was a Tauranga City councillor before entering Parliament in 2014, was in charge of the city's Straight Shooters Bar in 1997 when a gang member with facial tattoos was refused entry.
"It turned into a confrontation, a physical one. He was a lot bigger than me, he was a very intimidating individual. I got a black eye and swollen face out of it. He picked me up above his head and tried to throw me across the front entranceway, but I held on to his belt and got myself to the ground."
A few years before the fight, a close family friend working at an Auckland bar had his face blasted with a shotgun after refusing gang members entry, Mitchell said.
"I followed him [the shooter]. I told the staff to call the police, which they did. The police arrived, and, by the time they had arrived, I'd run up and gave him a beating, basically, gave him a bit of a boxing lesson.
"Had I just repelled him at the front door and left it, then he would have been arrested and there would be no charges against me."
The other eight NZ First MPs have been confirmed as party candidates for the 2020 election.
They are Peters, Darroch Ball, Shane Jones, Jenny Marcroft, Ron Mark, Tracey Martin, Mark Patterson, and Fletcher Tabuteau.
