David Boag, 26, had been working at a council-run facility when he put his mobile phone under a cubicle door after seeing the woman enter a changing room.
However, she spotted the device and screamed before opening the door to find Boag looking at her.
She said she recognised him as a staff member at the South Lanarkshire Lifestyles facility in Lanark.
The incident was reported by the woman, who cannot be identified for legal reasons, and an investigation was launched.
Boag, of Larkhall, Lanarkshire, faked illness when he was confronted by senior staff but was arrested following a police investigation.
He has now been convicted of using his mobile phone with the intention of filming the woman while she was undressed in July last year.
A trial at Lanark Sheriff Court heard Boag deny the allegations and claim he had been in the changing area checking a faulty tap.
He said: “I have no recollection of that happening so I can’t say anything about what that woman said.
“I honestly can’t tell you what happened or if I had my phone in my hand while I was in the changing area.
“I’d been in the changing area to see if I could fix a tap that was broken.
“I could have been in that area at that time but I have no recollection of her at all.
“If someone had shouted at me then I would’ve heard it because there is noise all the time in the changing area but I can’t remember anything causing me concern.”
Boag added: “I didn’t take a photo of this woman and didn’t delete anything from it before the police came.
“I have never been in trouble with the police before.
“I was stunned by what was being said to me after the manager spoke to me, I felt sick and had to bin my dinner because I couldn’t eat it.
“I didn’t know the woman who made the allegation against me, I can’t even remember seeing her that day.”
CCTV footage showed Boag walking through the leisure centre as he carried out his duties before the incident but filmed him looking noticeably panicked moments later.
Data from electronic door swipes also showed he had been hurrying through the centre as he tried to flee the changing area.
Centre manager Ann Hamilton told the trial: “When I told him the complaint had been made against him he was shaking and asked if he could get some water.
“When he had come back from his duties in the building he still had his phone in his hand but didn’t try and hide it or anything.
“He shouldn’t really have had his phone on him at all but there are times when I know staff do have their phones while at work.”
Boag was found guilty by Sheriff Nikola Stewart who told him he had committed a gross breach of trust.
She ruled he had operated the mobile phone with the intention of enabling himself to see the naked woman doing a private act by placing it under the cubicle door.
Sentence was deferred for reports and Boag was given bail and placed on the sex offenders’ register.