Full-time from the Proact Stadium
Shane Ferguson fired The Lions into a 13th-minute lead before Morison took centre stage with two goals in the space of three minutes just before half-time.
Although Chesterfield reduced the arrears on 65 minutes with a Sam Hird header, the damage had already been done and it was The Lions who ran out worthy winners on the afternoon.
In an explosive opening 90 seconds both teams could have taken an early lead.
There were just 40 seconds on the clock when Millwall midfielder Shaun Williams picked up the ball on the edge of the penalty area, nonchalantly swung his left boot and hit a crisp drive that rattled the crossbar with keeper Ryan Fulton grasping at thin air.
Play quickly swung up the other end and when Ched Evans mis-hit a Jay O'Shea cross, Gboly Ariyibi's low drive was saved by Jordan Archer.
Both teams certainly looked enterprising in the opening exchanges, but it was The Lions who made the all-important breakthrough with 13 minutes on the clock. A delightful move involving Morison, David Worrall and O'Brien sliced through the home defence as play swung from the right to left flank where Ferguson picked up possession, strode forward and struck a low angled drive into the bottom corner.
Chesterfield were a whisker away from conjuring up an instant equaliser, but to the relief of the 500+ travelling fans camped behind the North Stand goal, Dan Gardner's stinging drive flew past the post
The Lions had a great chance to double their tally in the 16th-minute when a delightful Ferguson cross picked out an unmarked Worrall at the back post, but the winger could only direct his header wide of the target.
Moments later O'Brien flicked the ball into the path of strike partner Morison who cushioned the ball and hit a first-time shot that, whilst on target, was comfortably collected by Fulton.
The Spireites then enjoyed their best period of possession in the opening half and it took some sound defensive work from skipper Tony Craig and left-back Joe Martin to make timely clearances from Evans and Ariyibi in quick succession.
Harris' men continued to look a threat going forward and duly forged 2-0 ahead four minutes before the break. Byron Webster showed great vision to intercept the ball from Evans on the halfway line, took a couple of touches then threaded a sublime ball through to Morison who hit a thumping 18-yard shot beyond the reach of keeper Fulton.
The Lions were forced to make a change moments later when Martin, who had received treatment minutes earlier following a Millwall corner, was forced off and Fred Onyedinma came on. A defensive reshuffle saw goalscorer Ferguson drop to left-back and Onyedinma take his place on the flank.
It didn't take long for the visitors to add a third goal and by the 41st-minute Morison had grabbed his second of the game with an exquisite lob after an intelligent forward pass from Williams had caught the home defence napping.
With the fans in full voice throughout the first 45 minutes and Millwall 3-0 up at half-time, on paper, it should have been a formality for Harris' side to see the game out and claim three valuable awayday points.
Having let a two-goal lead at MK Dons slip earlier in the season, The Lions were in no mood to relinquish either their advantage or the points, but did have to reshuffle the defensive pack yet again when sub Onyedinma came off injured. On came Sid Nelson to take his place in the heart of defence in a move that saw Craig switch to left-back and Ferguson revert to his place in the wide midfield berth.
Unsurprisingly the home side looked to get an early foothold back in the game and Archer had to be alert to deal with long-range efforts from Ian Evatt and Evans, but neither really troubled the Millwall keeper.
The Lions continued to be a real handful on the counter attack and O'Brien would have been clear through on goal if he had not been impeded by a Chesterfield defender, but referee Lee Probert inexplicably waved play on.
However it was the home side who pulled a goal back on 65 minutes courtesy of Hird's firm header from Jon Nolan's dinked free-kick to give the Spireites hope of a remarkable fightback.
What was remarkable as the game progressed was the failure of the floodlights to be switched on despite the fading light, dark clouds and subsequent downpour of rain.
The mood in the Millwall camp darkened further when Ferguson became the third visiting player to hobble off with an injury, although it did hand an opportunity for new signing Calum Butcher to make his first appearance following his move from Burton Albion 48 hours earlier.
Chesterfield tried in vain to add to their solitary one goal tally but were thwarted by a dogged, determined Lions side who were excellent value for their victory.
Scorer: Hird (65mins)
Booking: Archer (67mins), Williams (70mins)
Scorer: Ferguson (13mins), Morison (38,41mins)
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