Simpson, who made his debut against Barbados earlier this year, played against The Lions during their Summer Tour of British Columbia and had been plying his trade with Portland University when Wise made his move. The 21-year-old left winger, who can also play in defence, has signed a two-year deal and goes straight into the First Team squad for this Saturday's Championship opener against Plymouth Argyle.
He follows in the footsteps of former Millwall goalkeeper Kasey Keller, who also signed for the club from Portland University, way back in 1992.