Mired in the Championship relegation zone at the start of the month following a run of five consecutive losses, Saturday's 2-1 success means the Tigers now sit five points clear of the bottom-three and are starting to look very much at home in the second tier.
They certainly did not have things all their own way against a Lions side who harbour realistic play-off ambitions, but having twice got themselves into a winning position, Grant McCann's men showed resolve and desire in abundance to see the game out.
The first decent opening of the afternoon arrived on nine minutes when Benik Afobe climbed well inside the City box to meet a cross from the right, but powered his header over the cross-bar.
The same player then stung the palms of Hull stopper Nathan Baxter, who made another decent save just moments later, getting down to his left to push away Jed Wallace’s low strike from the resulting corner.
Having failed to really open up the visiting defence despite engineering some decent attacking opportunities, City then edged in front just before the half-hour mark.
A decent bit of football afforded left-sided centre-half Jacob Greaves the opportunity to join in on the overlap and his inviting cross found George Honeyman, who nodded home from inside the six-yard box.
The Tigers looked comfortable at 1-0, but were stung on the counter-attack with just seconds of the opening period remaining as Tom Bradshaw beat the offside trap, ran clean through on goal and confidently slotted a finish beyond the advancing Baxter and inside the near post.
There was still time for Hull to reclaim the ascendancy in stoppage-time, however Mallik Wilks’ instinctive effort from Keane Lewis-Potter’s knock-down had too much on it.
But the home side did add a second just eight minutes after the resumption after Millwall failed to clear their lines from a right-wing corner.
The ball eventually broke for Josh Magennis, whose low strike on the spin was parried by Bartosz Bialkowski, but only as far as Ryan Longman, who pounced on the loose ball.
Millwall heads did not drop, and they could easily have been level when Afobe fired the ball across the face of goal on 61 minutes, but with the goal gaping it was a City defender who made first contact and cleared the danger.
An incisive move down the visitors' right then saw George Saville send full-back Danny McNamara one-on-one with Baxter, the Hull custodian standing tall at his near post and blocking well.
From the resulting corner, Baxter flung himself to his right to keep out a goal-bound header, Honeyman then arriving in the nick of time to hack the loose ball to safety.
With Hull dropping ever deeper and looking to pick the Lions off on the counter-attack, it was the away team that continued to look the more likely during the closing stages though Baxter was not seriously extended again as those in front of him threw their bodies on the line.