For the second successive home game, Whites supporters gave the man in the middle a barrage of abuse, with referee Jeremy Simpson ridiculed for large parts of proceedings, compounded by the decision to award Brentford a penalty just after the hour mark.
Ollie Watkins went down under minimal contact from the onrushing Bailey Peacock-Farrell, with replays showing that the Bees player had blatantly dived.
Neal Maupay coolly sent Peacock-Farrell the wrong way before unwisely celebrating in front of the South Stand, with a number of objects thrown in his direction.
It was a tough development for Leeds, whose head coach Marcelo Bielsa cut an agitated figure on the sidelines.
Succour finally arrived when expectant father Jansson headed a late leveller, but there was still time for one other development when Luke Ayling was dismissed for a second caution.
On the day, Leeds were seocnd best, but at least showed character to take away something from a problematic afternoon.
Widely perceived to rank alongside Leeds as the best footballing side in the league, Brentford arrived in West Yorkshire with a fair reputation and showed evidence of just why, particularly in the first half.
Statistical form on the road may not have been great, with the Bees without an away win since April 10, but their prowess on the ball was clear to see with the Londoners looking the much more fluid in the first half.
The likes of Sergi Canos and Ollie Watkins posed problems with their movement while holding midfield duo Nico Yennaris and Kamohelo Mokotjo ensured that Leeds' own creative department of Samu Saiz, Gjanni Alioski and Jack Harrison were unable to dictate the tone of the game for the hosts.
After his costly error in the previous home game with Birmingham, Bailey Peacock-Farrell enjoyed a far more rewarding half, making a key finger-save save to turn away an early goalbound low shot from Neal Maupay, which was otherwise destined for the far corner after Leeds were opened up.
The United keeper also made an equally important block just before the half-hour mark to deny nine-goal Maupay from close range after Watkins' goalbound shot took a deflection and clipped the bar before bouncing back into play.
The slick play largely arrived from Brentford, but Leeds went close before the break when Saiz fired over after a smart passage of play.
Leeds' best other chances saw Luke Daniels gather an angled low drive from Tyler Roberts, while Mateusz Klich, who lost a contact lens partway through the first half, saw a strike from distance flash wide.
Brentford started the second half in the same vein as the first half and troubled Leeds from an early juncture.
A great early free-kick opportunity was wasted by Watkins after Jansson was penalised for his challenge on Canos, with the Swede then showing immaculate timing to block a drive from the influential Bees winger soon after.
Second best on the day, but with the scoreline still in the balance, Leeds soon started to assert themselves and went mighty close to an opener when Kalvin Phillips' bundled rebound from close range was splendidly turned onto the post by Luke Daniels.
The Bees keeper - who had initially failed to grasp Liam Cooper's header following Alioski's deep corner - then gathered the loose ball.
Controversy soon reigned in a major way with Watkins' went to ground after Peacock-Farrell with considerable salt rubbed into United's wounds when Maupay sent the home keeper the wrong way to put the Londoners ahead.
The Elland Road crowd was fuelled with injustice and a famous leveller almost arrived when Cooper nipped in front of the advancing Daniels following Alioski's inswinging free-kick, only for his header to drift agonisingly wide.
Tempers began increasingly raised with Roberts hooking wide for Leeds, who then went close to a leveller when a wonderfully whipped free-kick from substitute Lewis Baker flew just wide after Mokotjo was penalised for a challenge on Alioski.
Brentford had opportunities to seal it on the counter, but Leeds hung in there and was rewarded when Jansson headed home in front of the Kop following Alioski's free-kick on the left.
Baker went close to adding a late winner for Leeds, glancing just off target before Ayling was dismissed for a second caution.
Leeds United: Peacock-Farrell; Ayling, Jansson, Cooper, Dallas (J Clarke 70); Phillips; Harrison, Saiz (Baker 75), Klich (Forshaw 63), Alioski; Roberts. Substitutes unused: Blackman, Pearce, Shackleton, Edmondson.
Brentford: Daniels; Dalsgaard, Konsa, Mepham, Odubajo (Barbet 76); Yennaris, Mokotjo; Watkins (Judge 84), Sawyers, Canos (Benhrama 76); Maupay. Substitutes unused: Bentley, MacLeod, McEachran, Jeanvier.
Referee: J Simpson (Lancashire).