The Englishman, who is preparing for Euro 2012, was given the award in a month that saw him score against Swansea in a 2-0 home win in a vote on the club's website
The 26-year-old started in both of the Red Devil's games in May against Swansea and Sunderland.
The winger, who is currently training with the England squad ahead of Euro 2012, scored against the Swans in United's 2-0 win and was a constant threat in his side's last-day 1-0 victory at the Stadium of Light.
His manager Sir Alex Ferguson was full of praise for the ex-Aston Villa man who signed at Old Trafford last summer.
Sir Alex told the club website: "Ashley scored eight goals which is a good contribution in his first season here.
"He has a goal in him. There's definitely a goal in that lad. He's a good finisher."