The Gunners matched the reigning European champions for the best part of 60 minutes at the Emirates Stadium, but soon succumbed to a 2-0 humbling.
Lionel Messi opened the scoring after 71 minutes before adding a second from the penalty spot 13 minutes later, making Barcelona huge favourites to reach the quarter-finals.
And Mascherano says his side were aware that Arsenal would tire towards the end, presenting Luis Enrique's men with the opportunity to pounce in the closing stages.
"We knew that Arsenal couldn't maintain the intensity of the first few minutes," Mascherano was quoted as saying in the Mirror.
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"We knew that sooner or later we would find them disorganised at the back.
"And in the end that’s how it was. We seized the moment and we've taken a great victory that allows us to face the future with greater optimism."
Barcelona defend a two-goal lead in the return leg at Camp Nou on March 16, but Mascherano has warned they cannot afford to relax just yet.
"Clearly we've done a good job but I still don’t think it's definite," he added.
"Arsenal are a team with a lot of quality that can make life difficult for anyone."