Francois Olwage was caught by his colleagues, who posed as the girl.
After messaging her for two weeks, he was arrested as he bought an ice cream for her just before they were due to meet in Basingstoke.
At the time Olwage, 52, a Met Police counterterrorism specialist, was meant to be working from home in Stevenage.
He was found guilty of three charges including attempting to incite a girl aged 13 to engage in sexual activity.
He has lost his job and will be sentenced at Winchester Crown Court on April 27.
