The centre-forward set his side on their way to a 2-0 lead only to lose his head once Matty Cash halved the deficit.
Hull manager Grant McCann billed the game as a test of character after back-to-back defeats and he must have been delighted with how tigerish his Tigers were after Magennis’s rashness forced them to play a quarter of the game with 10 men.
Their 4-2-3-1 formations might be the same, but Hull and Nottingham Forest play it very differently and the old-fashioned grit of the East Yorkshire side came in very handy against Sabri Lamouchi’s sophisticates as they moved up to 14th in the Championship.
Forest’s is very much the fashionable approach, full-backs pushed on, wide men took inside to open up space and holding midfielders often auxilliary midfielders. In the opening stages it looked like it would hold sway but this has been a bad midweek round for the leading promotion contenders and McCann’s slightly more prosaic side had other ideas.
Joao Carvalho and Sammy Ameobi both gave the visitors early warnings, the No10 twinkling his toes before curling a shot wide of goal, the gangly but speedy wide man putting Eric Lichaj on the ground with a shimmy only for Jordy de Wijs to cut out the cross.
In Magennis and unused substitute Tom Eaves Hull have two big target men centre-forwards, and Kamil Grosicki and Jarrod Bowen are wingers rather than wide attackers. As the half wore on, the Tigers imposed themselves.
Jon Toral had a shot saved after 19 minutes but an off-balance Bowen put his shot over when he drifted in off his right flank.
Bowen ought to have done better on the half-hour, Grosicki exploiting left-back Jack Robinson’s wanderlust to play his team-mate into time and space but Brice Samba saved.
Forest were still having a say, Ameobi chasing a lost cause and back-heeling the ball before it went out for a goal-kick to instead win a corner. Grabban dragged a shot wide after a possible foul on Toral in the build-up.
The centre-forward headed over from another corner won by Thiago Silva’s exquisite pass and Robinson’s cross.
But the balance of play had shifted and Hull made it count in the 38th minute.
Toral and Magennis were two of four changes by McCann and they combined to put their side in front.
Whether it was excellent goal-poaching or sheer luck, Magennis followed his first league goal for Hull on Saturday with his second, turning in Toral’s shot.
Just listening to the crowd told you all about Forest’s frustration.
Hull were quick to add to it after the break, with Bowen pivotal.
A volley from Grosicki’s pass forced Samba into a save then, a minute later, Bowen gave the goalkeeper no chance as Hull struck on the counter-attack.
Grosicki released Toral who cleverly drew his man, then squared for a chance Bowen was never likely to pass up.
Forest had cut the lead within five minutes, right-back Cash running relatively unfettered into the inside-left channel and sliding his shot into the net.
After wobbling a little, Hull regained their composure, only for Magennis to lose his as the crowd played its part.
Oliver Langford waved play on when Lichaj got a crucial toe on the ball in the penalty area but the crowd were in uproar as Ameobi beat the turf, arguing the defender had gone through him.
Magennis’s temperature rose too, launching into a terrible tackle on Ben Watson which saw him red-carded by Langford for the second time this season.
Toral, who had been getting a surprising amount of space in the hole, was briefly pushed up to centre-forward, then replaced by debutant Norbert Balogh. The more pragmatic George Honeyman replaced Grosicki as Hull attempted to dig in.
It made for a tense finale with Long and Bowen both booked for time-wastin, although the Hull back four kept the goalkeeper’s workload to a minimum.
Long did have to scramble the ball around the post, though, when Thiago Silva opted to shoot from a free-kick out wide, before Joe Worrall headed wide.
Cash dragged a shot wide as Forest camped in Hull terrirtory without hitting the target often enough. Joe Lolly looked as if he was going to do just that, only for the ball to hit one of the massed ranks of defenders between him and the goal.
Nottingham Forest: Samba; Cash, Worrall, Rodriguez, Robinson (Mir 80); Watson, Silva; Lolley, Carvalho (Yates 61), Ameobi (Adomah 73); Grabban. Unused substitutes: Robeiro, Figueiredo, Johnson, Muric.
Hull City: Long; Lichaj, Burke, de Wijs (Tafazolli 65), Elder; Lopes, Irvine; Bowen, Toral (Honeyman 73), Grosicki (Balogh 73); Magennis. Unused substitutes: Stewart, Eaves, Ingram, Bowler.
Referee: Oliver Langford (West Midlands)