The Canaries were trailing to a first-half Ivan Toney goal, his eighth in six Sky Bet Championship matches.
But for the fourth match running, Norwich struck with time running out as McLean’s deflected shot salvaged a draw.
Norwich should have opened the scoring after a superb piece of trickery from Emi Buendia.
But it was Brentford who took the lead after a lightning counter-attack from their new-look strikeforce.
Ghoddos burst forward before feeding Mbeumo, who reached the byline before cutting the ball across goal for Toney to slide home.
Toney almost had a second shortly after but Tim Krul dived full length to claw away his header, with Mbeumo putting the rebound into the side-netting.
Norwich nearly grabbed a leveller before the interval with Grant Hanley heading Buendia’s cross inches too high.
After the break Brentford were denied a second when Toney’s header back across goal found Dalsgaard right in front of goal but the defender’s finish was too close to Krul.
But as they chased an equaliser, Buendia’s attempt to bend it like Beckham from the edge of the box resulted in a complete air-shot.
However, Norwich have developed a happy knack of scoring last-gasp goals this season – including winners in their last three matches.
And with three minutes remaining, McLean’s speculative drive was going wide until it deflected off Mads Bech Sorensen, wrongfooting Raya as the ball rolled into the net.