The opener took his chance to impress ahead of Saturday's opening Bob Willis Trophy clash against Warwickshire at Edgbaston, scoring 69 from 114 balls.
Curran hit eight boundaries and two sixes against a Middlesex attack that included former England fast bowler Steven Finn.
It wasn't initially such a good day for Curran's fellow batters Emilio Gay, Rob Keogh, Charlie Thurston and Ricardo Vasconcelos - although the latter did return for a second innings later in the day to notch a half-century himself.
Gay and Keogh both also had two bites at the cherry, but they failed to take either of them, with Keogh suffering a golden duck first time out when he missed a full and straight one from Finn.
Vasconcelos didn't miss out in his second dig though, making 61 from 66 balls, while Thurston also went back out and made an unbeaten 35 as Northants ended their 90-over day on 355 for nine.
Skipper Adam Rossington chipped in with 31 while Saif Zaib impressed before retiring on 48, and Luke Procter made 35 before he too voluntarily returned to the pavilion.
The match was the first red ball match to the played at Wantage Road since Northants beat Durham in the Specsavers County Championship Division Two last September.
The 2020 season has been suspended since April due to the Covid-19 pandemic, but the campaign begins this weekend.
The friendly with Middlesex has been arranged to give people time in the middle, with Northants batting all day Tuesday and Middlesex batting all day tomorrow.
The pick of the bowlers for the Londoners was Finn, who ended the day with six for 58 from 18 overs.
Curran was pleased with his day's work, saying: "It was good to get some time in the middle, it’s been a long wait for everyone and it was nice to get some cricket, nice to be back out there.
“I’m pretty happy, I felt like I played nicely and there’s nothing that compares to game time and competitive cricket. Middlesex are a good team, so it was nice to get a few runs.”
