The 27-year-old, aiming to retain his place for the Test series against Sri Lanka later this month, hit 10 fours and a straight six off Ollie Rayner.
Toby Roland-Jones bowled Hales as he attempted a drive, ending a 107-run third-wicket stand with Michael Lumb.
England seamer Steven Finn then claimed two late wickets as Notts closed on 345-7, with Samit Patel well set on 86.
Finn, who is looking to return to the England set-up after injury, struggled with his pace early on and was wayward, leaking 46 runs from his first eight overs.
In contrast, Tim Murtagh, who trapped Steven Mullaney lbw with the fifth ball of the match, and Roland-Jones - who also claimed the wickets of Greg Smith and Riki Wessels - made good use of the ball in the morning session.
Lumb's valuable 78 was ended when he was trapped in front by Rayner, but Patel followed up with his third half-century of the Championship season.
The second new ball brought joy for Finn, as the extra lift saw Chris Read caught by Nick Compton at gully and then Stuart Broad edge to third slip in his next over.
But Patel remained in an attacking mood, dispatching the Middlesex bowlers to the boundary 11 times as he finished just 14 short of his century.
England opener Alex Hales made an entertaining 73 for Nottinghamshire against Middlesex on day one at Lord's.