The 20 year-old has signed a one-year deal with the Bees with a 12 month option.
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McKnight won his contract after impressing in preseason trials.
Brentford B head coach Sam Saunders told the club's website: "We’re really excited to have Maxwell on board. From the first couple of days we saw him, it was clear he could add real quality to the group.
"He adds competition to the team and has a versatility that will make him a valuable member of the squad. He is able to play at left-back, right-back, and also in midfield.
"We’re looking forward to helping him develop throughout the season as we continue to get better and improve as a team."