PC Swaleh Chaudhry, 36, was caught out when the victim spotted his phone being used to spy on her at the store in the Southside Shopping Centre in Wandsworth.
After he was arrested, officers found a cache of images of children being sexually abused at his home, with infants said to be among the victims.
At Kingston crown court on Friday, Chaudhry was sentenced to ten months in prison suspended for 18 months.
He has been held in custody since his first court appearance in April, but is not set to be freed.
Scotland Yard said Chaudhry resigned from the Met in the wake of his arrest. “Former Constable Chaudhry’s behaviour was repugnant”, said Commander Kyle Gordon, who leads the Met’s Taskforce.
“His actions go against everything we stand for.
"Holding the office of constable is a privilege and it comes with great responsibility. He showed himself to be totally unsuitable to hold this office and undeserving of the trust of the public, and it is right that he has resigned.”
At his first court hearing, Chaudhry pleaded guilty to three charges of making indecent images of children, possession of extreme pornography involving animals, and voyeurism.
“A female complainant was in the fitting room trying on clothing and she stated she heard rustling in the fitting room next to hers”, said prosecutor Suleman Hussain.
“Under the benches was a gap in between the fitting rooms, she saw a black mobile phone with the camera facing her.
“She saw the camera appearing to follow her as she was moving, she began to scream ‘are you recording me?’.
“There was a response from the cubicle next to hers, saying no. She came out and warned her boyfriend, and the defendant was then challenged.”
The court heard Chaudhry was detained by shop security on March 30, and a later search of his Wandsworth home by police uncovered more than 1,000 images of children being sexually abused including at least 50 in the most serious category.
Chaudhry says the images were downloaded in 2012 and 2014, and he wants to seek treatment. He has been handed a Sexual Harm Prevention Order.