The ex-Forth Wanderers boss and his assistant Gary Flint depart only two weeks before United begin their West of Scotland Conference C campaign with a home game against Larkhall Thistle at Moor Park on Saturday, July 17.
“Pre-season’s been really difficult,” McKenzie told the Carluke and Lanark Gazette.
"We’ve really struggled to get our main targets in the door and that has been a sense of frustration for us.
"We had a number of targets in mind and even back-up targets when the first choice hadn’t come through when we couldn’t get people tied down and get them to commit.
"And there was a lack of commitment from the squad we had. There was obviously work, holidays and stuff which reduced the numbers greatly in training.
"I just felt for us to go there, be professional and try to work, it wasn’t the way we wanted it.
"It’s not going the way we had planned it with the team going forward. We are two weeks away from the season starting so it’s not ideal at this point in time but I think we had to make the decision.
"I just felt it was the right decision both for myself and for the club. I have decided to call it a day and let someone else come in.”
Jamie’s decision to quit Lanark was accelerated by the postponement of last Saturday’s scheduled home friendly against Blantyre only the day before.
This was off because five Lanark players were in self isolation and others were unavailable due to injuries or work commitments.
"We had 12 players of a squad unavailable that were meant to be available on Wednesday when we finished training,” Jamie said.
"We had a squad of 19 going into the game and then by Friday morning we had 12 call-offs.
"I was running about like an idiot trying to put a team on the park stressing myself.
"The boys who were isolating, that’s fine, it needs to be done and we can’t take risks. And nothing really could be done about the injuries.
"But, with people working, if you commit to football I’m of the opinion that you make yourself available for every training session, every game.”
"I think certain people will be happy I’m away now and that’s fine.
“But one thing I would like to do is emphasise my thanks to George Reid (United president) who’s been mega supportive and if only the rest of the committee were the same as him then it might have worked out differently.
"I worked closely with George and his help and support was greatly appreciated.
"We had lost seven or eight of our better players from last season.
"I’ve only really been in charge for six or seven games and I think we’ve only lost two.
"We weren’t a million miles away. We have played three games pre-season and we have won two of those.
"Granted, we lost heavily in the other one but we still believed we could turn it around.
"But just the way it was developing I didn’t like and we decided it was time to go.”
Dad of four McKenzie, who works as a council sub contractor, will now take some time away from football but would like to return to the game in future.
"Since Sunday I’ve had two job offers to go and coach at other clubs which is great to have,” he added. “But it’s not for me at this point in time.
"I’m quite busy with my sub contractor job. That will keep me busy along with my four kids, four boys, so we’ll concentrate a bit on their football as well and get to enjoy watching them.”
With McKenzie gone, former Lanark manager Colin Slater is taking club training tonight (Monday).
Slater will be interim gaffer initially and is likely be a contender to take the job on a permanent basis.