The young Bees fell behind early against the run of play before they levelled through Mogensen’s curled effort. But a tough first half saw them concede three more before the break.
The Danish striker netted a superb free-kick in the second half but they were unable to get themselves back into the match in Salzburg.
It was the young Bees who started the brighter with Neil MacFarlane’s side looking dangerous down the left-hand side.
An early attempt for Nick Tsaroulla which edged just wide of goal preceded the first goal of the game against the run of play. As Red Bull broke forward, they were awarded a corner which was whipped in dangerously before Ahmet Gureyen rose highest after getting the wrong side of the Brentford defence to head home from close range.
Despite that, The Bees continued to push forward and levelled shortly after falling behind when Jaakko Oksanen drove forwards before playing a fantastic ball out to the left into the path of Mogensen. The young Dane controlled well to drive towards goal and curl into the far corner.
After working so hard to get themselves level, The Bees would be disappointed to concede another just five minutes later when a misplaced pass in the back line gave Csaba Bukta the chance to nip in and drive at goal before slotting the ball past Shepperd with composure.
Brentford B continued to press forward and Fredrik Hammar had a speculative effort go over before they were given another sucker-punch when good play down the left-hand side from the home team saw the ball threaded through to Tobias Anselm who chipped the ball in despite the protests for offside as the score became 3-1.
Mogensen would have another attempt but this time saved after good initial link up play with Halil Dervişoğlu, whilst David Titov combined well with Matej Majka before his dangerous cross was swept away by the Liefering defence.
They would regret their missed opportunities even more when the home side added a fourth goal after excellent solo ability from Maurits Kjaergaard who beat a number of players before he dribbled his way into the box and finished calmly.
Credit to the young Bees who came out after the break with a determined attitude to wrestle themselves back into the game as Oksanen stung the palms of the Liefering keeper with a free kick before Jan Zamburek’s teasing cross just evaded Mogensen at the far post.
On the hour mark the young Bees would have the ball in the net again after they recycled a corner well before Hammar turned expertly to finish low into the right-hand corner, but the referee ruled out the goal, stating there was a foul in the build-up.
That second Bees goal did come, though, on 70 minutes when they were awarded a free-kick just outside the Liefering box and Mogensen stepped up to arrow a wonderful effort into the top corner leaving the Liefering keeper with little chance of saving as the score became 4-2.
Just one minute later the home side would have a free kick of their own and Alexander Prass attempted to recreate Mogensen’s magic with another powerful effort, but he was only able to crash his attempt against the crossbar.
Despite a battling performance, Brentford B were unable to get themselves back into the match as the game finished 4-2 to the hosts.
FC Liefering: Stejskal, Windhager, Tijani, Kjaergaard, Oroz, Major, Anselm, Gazibegovic, Aigner, Bukta, GurleyenSubs: Schrocker, Dedic, Okoh, Adamu, Seiwald, Sesko, Sucic, Prass, Barry
Brentford B: Shepperd, Tsaroulla, O’Connor (Mitchell, 82), Sery Larsen, Oksanen, Majka, Hammar (Carre, 82) Zamburek (Tue Jensen, 75) Dervisoglu (Pressley, 57) Mogensen, Titov (Adams, 61)Subs not used: Andersson