First-half goals from Colombian international William Tesillo and Ecuador forward Angel Mena looked to have secured Leon a comfortable victory at the Estadio Leon.
But the Liga-MX side's hopes of winning the continental title for the first time were jolted after Los Angeles forward Denis Bouanga scored in the sixth minute of second-half stoppage time.
Los Angeles coach Steve Cherundolo admitted his team had been lucky to escape with just defeat by a one-goal margin and vowed an improved performance in Sunday's second leg in California.
"Obviously it's a bit of a lifeline. That goal does give us a chance. But that goal alone won't help us win the final. The performances as individuals and as a group needs to be much, much better," Cherundolo said of Bouanga's late strike.
"The scoreline is extremely lucky for us. But we're experienced enough and smart enough to bounce back. I'm 100 percent positive that our performance on Sunday will be much improved."
Los Angeles went into the first leg with high hopes of emulating Seattle, who last year became only the second MLS team to be crowned champions of CONCACAF, the regional grouping for teams from North America, Central America and the Caribbean.
However the California club were given a footballing lesson for long periods by an incisive Leon team who had not played since a 7th May defeat in Liga-MX.
June 1, 2023
Leon dominated the first half with the movement of their front line, led by Jose Alvarado and Victor Davila, repeatedly exposing Los Angeles down the flanks.
Leon did not have to wait long for the opener. A corner by Mena was met by left-back Tesillo, whose angled header flew in off the post to make it 1-0 on eight minutes.
Los Angeles almost got back into the game in the 36th minute but a promising counter-attack broke down when Jose Cifuentes misdirected his pass into the penalty area behind advancing teammate Kwadwo Opoku.
Leon's hopes were then given a huge boost in stoppage time at the end of the first half when defender Jaine Barreiro's header from a corner was adjudged to have just grazed LAFC fullback Ryan Hollingshead's forearm.
Referee Walter Lopez initially gave a corner, but after a review by VAR the Guatemalan official pointed to the spot to roars of delight from the home crowd. Mena stepped up to hammer home the spot-kick to leave Leon 2-0 up at half-time. Los Angeles rallied briefly early in the second half, but again failed to seriously test Leon goalkeeper Rodolfo Cota.
Instead it was Leon who looked the likelier to score, with only LAFC goalkeeper John McCarthy keeping the tie alive with a string of fine saves, including one remarkable 58th minute block to deny Mena from close range.
Leon looked to have killed the tie off in the closing stages but had a goal disallowed. Instead Los Angeles finally conjured a breakthrough in the dying seconds when Bouanga prodded home Mateusz Bogusz's cross.
June 1, 2023