They believe the affair was motivation to murder his wife, that’s why they called Gypsi Willis to the stand Friday.
It began with her disclosing she is testifying in exchange for getting no jail time in another felony case.
She then went on to say the two met in November, 2005.
"We met online," said Willis.
Trying to build their case for the jury, prosecutors had her lay out the nature of their relationship. Willis testified it turned sexual in January, 2006.
"I think we probably had sex half the time, I mean sometimes it was just lunch," said Willis.
Willis testified that in March, 2007 she moved into a duplex in Lehi, paid for by MacNeill.
She said he also provided her with a debit card for expenses.
The prosecutor asked her if she knew what happed on the day Michelle MacNeill died, April 11th, 2007.
"I understand that Martin's wife was found collapsed. She was taken to the hospital, coded and then passed away," said Willis.
They broke down phone records, revealing several calls and texts between the two around that time, including the day of Michelle's death and funeral.
Then it turned to an encounter, they say MacNeill set up a week later with his family at the temple.
The prosecutor asked, "what were you supposed to do when you went to the temple?”
“I was supposed to walk up and introduce myself and maybe strike up a small conversation and leave, just an introduction," said Willis.
In the middle of her testimony the judge called for a break and sent the trail to recess for the weekend.
It will resume, with Willis on the stand, on Tuesday at 8:30 AM.