Watkins had a great chance to give the home team the lead straight away in the match, but John Victor in the Nottingham goal made the save.
Instead, Watkins had to wait until first-half stoppage time to strike, when he finished from distance with a powerful shot that John Victor couldn't reach.
The 30-year-old has now scored six goals this season.
Early in the second half, John McGinn added to the tally after a fine Villa attack. The Scot then became a two-goal scorer when he was able to take advantage of John Victor being off his line.
The Nottingham goalkeeper injured himself in the goalmouth incident and left the field in tears.
Morgan Gibbs-White pulled a goal back for the away team, but they didn't get any closer than that.
Nottingham finished seventh last season and secured a European spot. The team is now in 17th place in the Premier League.
With the win, Aston Villa move above Manchester City into second place in the table, although City have a game in hand.
