0948 GMT December 14, 2019
Olaf Cunitz, the city's head of planning, said the soldiers were believed to be from Napoleon's Grand Army retreating from Russia in 1813.
He said they had probably died from battle wounds or from typhus, BBC reported.
Battles fought near Frankfurt in 1813 claimed 15,000 lives, Cunitz added.
The skeletons were found in the city's western Roedelheim district.
Andrea Hampel, heritage and historic monuments director for Frankfurt, said it appeared that the mass graves were dug ‘in an emergency’.
She said that the victims had been buried in coffins which were aligned in a north-south axis, rather that the traditional east-west orientation, suggesting they were buried in haste.
More than 30 skeletons have been excavated so far and work on the others is expected to take four to six weeks.
Napoleon's 600,000-strong Grand Army invaded Russia in June 1812 and took Moscow in September. However, it suffered catastrophic losses and was forced to retreat. Of the original invasion force, only about 90,000 are believed to have made it back to France.