The victim said she felt “confused and shocked” after catching Benjamin McNish trying to take pictures of her naked on the morning of February 18, 2019.
The 30-year-old Metropolitan Police detective sergeant, from Benfleet, claimed he had been using the device as an “extension of my eyes” to find his razor after forgetting to shave ahead of a work drinks function.
But he was convicted of voyeurism, by observing a person doing a private act for his sexual gratification, by a jury at Southwark Crown Court.
McNish, a married father-of-two, who remains suspended, has a degree in Chinese and Spanish, speaks seven languages and was promoted to sergeant just three years after joining the Met.
But he is now almost certainly facing the end of his police career and could be sent to prison when he is sentenced by Judge Jeffrey Pegden QC on July 15.
A count of possessing an extreme pornographic image, which did not form part of the trial, was left to lie on file.
Crown Prosecution Service (CPS) senior specialist prosecutor Andrew Levin said: “Benjamin McNish was a serving policeman who observed a woman showering without her consent for his own sexual gratification.
“This was a private act that McNish had no right to observe. He violated the victim’s privacy by spying on her in a bathroom. His actions were clearly illegal and wholly unacceptable.
“During the trial, McNish claimed that he had just been using his phone to look for his razor by the sink, but the prosecution was able to prove to the jury that this was an absurd lie.
“The CPS will always aim to prosecute those who commit criminal acts, regardless of who they are, where there is the evidence to do so.”
McNish had been involved in reorganising the Met’s child abuse unit and was due to start a new role in the force’s rape and sex offences investigations squad at the time of the incident.