BERLIN—Andreas Lubitz, the Germanwings co-pilot who crashed an airliner into a French mountainside, was being treated for depression, a fact he concealed from his employer, according to a person familiar with the investigation.

Mr. Lubitz had been excused from work by his neuropsychologist for a period that included the day of the crash, this person told The Wall Street Journal, but he decided to ignore the advice and reported to…