The Lions headed into November in good form, but their chances were to take a setback following trips to Brentford and Norwich City.
A 2-0 loss at Griffin Park preceded a heartbreaking defeat at Carrow Road, in a game that saw The Lions lead twice - 1-0 and 3-2 - and Ben Amos save a penalty-kick from Teemu Pukki, but two late, late strikes from The Canaries saw the final result finish 4-3.
A battling 1-1 draw and a 2-0 Den loss to Birmingham City ended the penultimate month of the year, with December beginning on a Sunday afternoon in Bristol, as Shaun Williams' strike was enough to gain an important point at Ashton Gate.
Lee Gregory and Aiden O'Brien were then on target as The Lions and Hull City shared four goals and a point each, before narrow losses at Preston North End and Stoke City before Christmas made the festive period all the more important, with two home games and a trip to struggling Ipswich Town to follow.
Jed Wallace's long-range free-kick settled matters against The Royals on Boxing Day, whilst another Goal of the Season contender - Ryan Tunnicliffe's overhead effort - was enough to see off Nottingham Forest on 29th December.
The first day of 2019 then provided Millwall with another first - their opening victory on the road, as goals from Shane Ferguson (pen), Jake Cooper and Tom Elliott gave The Lions a 3-2 success at Portman Road.
A fourth win in a row was gained five days later, this time in the Emirates FA Cup, as Ferguson stepped off the bench to score a quick-fire double and send Harris' side into a Fourth Round tie with Everton at the expense of Hull.
A televised 2-0 defeat to Blackburn Rovers then sent everyone crashing back down to earth, but the good form continued, and if not for Jordan Hugill's 90th-minute penalty, Jed Wallace's first-half strike would have been enough to claim three more points, this time at promotion-chasing Middlesbrough.
On a rain-sodden Saturday evening in late January, the BBC cameras were present to see another Cup shock, as after Richarlison, Gregory, Cenk Tosun and Cooper had traded goals, Murray Wallace became the hero after turning home the winner in the 94th minute against Everton, putting The Lions into Round Five of the Emirates FA Cup.
Take a look at our gallery from the second third of the season above.