Chesterfield suffered a second consecutive defeat as Millwall ran out 2-1 winners on their first ever visit to the Proact Stadium.
The Spireites largely dominated the first half and took the lead after 25 minutes courtesy of a superb long range strike from Sam Morsy.
Their lead lasted less than two minutes, however, before Aiden O’Brien nodded home the equaliser despite the best efforts of goalkeeper Tommy Lee.
And the home side were made to rue missed opportunities as Lee Gregory’s goal early in the second period ultimately proved the difference to help the Lions move into the top six.