But while his first weeks with the Rossoneri saw him dubbed France's answer to Mario Balotelli, unlike his senior teammate, Niang has quickly heeded the advice of senior management and toed the line.
With the ink barely dry on his Milan deal, Niang was arrested by local cops in September for driving without a licence. The teen exacerbated the problem by claiming he was teammate Bakaye Traoré. Poor time-keeping and acting up while on France U21 duty just weeks later - it appeared Niang's Milan career would be over before it had really started.
But showdown talks with club vice-president Adriano Galliani did the trick.
"This is a very good player, young, who trains well," Galliani argued after the watershed summit in November. "But he needs to change his lifestyle outside of football."
For Niang, the penny dropped.
"The hard work is starting to pay off," says Niang. "There is a lot of competition, but I am never disheartened. At first I was a little silly, but I now understand how it works in Italy and I've calmed down.
"For me, I know I must pay attention to my lifestyle to avoid injury. At first, I had to prove myself all over again. And now I've begun to do so."
Niang's talent has never been in doubt.
At Caen he became the second youngest goalscorer in Ligue 1 as a 16 year-old. Relegation last season was considered a break for Niang. He'd be expected to play regularly in Ligue 2, so accelerating his development.
No shrinking violet, Niang already counts Milan president Silvio Berlusconi among his fans.
"I love this Niang. He reminds me of Balotelli but I hope he is better as a man," confessed Berlusconi.
The Balotelli comparisons won't go away. Niang is happy to be mentioned in the same breath as the former Manchester City firebrand and is currently sporting a similar Mohican hairstyle.
"Mario is a good guy, there is no problem with him. He makes you laugh, he does not make people cry," argues the youngster.
His former coach at Caen, Philippe Tranchant, is convinced the demands of being a Milan player will be good for Niang.
"He's never had to work so hard in his life. He eats and sleeps tactics. In Italy, the play in spurts, with a lot of one-on-one situations. This suits him well."
Tranchant keeps in regular touch with his former protege and adds: "His potential has never been questioned, but the problem is always M'Baye needs to be motivated. He can become distracted.
"He has a unique form of indifference, unlike most, he needs to be under pressure to do his best."
Niang's advisor, Khalid Ouadi, says his critics should remember the whirlwind he's experienced in the past eight months.
"Do not lose sight of all that has been compressed into his life: he has not been a teenager, he was immediately handed adult responsibilities and has already experienced things most 25 year olds have never known.
"So given everything that's happened, we must surely have a little patience."
That "patience" shown by Galliani and the Milan coaching staff is now being rewarded, with Niang proving himself a model pro.
Chelsea football director Michael Emenalo is aware of Niang's change of attitude and is now driving the prospect of luring him to Stamford Bridge.
While a new deal has been struck, the arrival of Balotelli does concern Ouadi.
Niang has only just extended his contract from 2017 to 2018, which includes a pay-rise from next season of €300,000 to €800,000. However, Ouadi admits he did speak with Milan directors in January after Balotelli's signing about a loan move.
"When we had the meeting to discuss a possible loan this winter, the leadership replied this was not under consideration and we have extended his contract.
"We discussed a lot at the time of the arrival of Balotelli."
For all the admiration he has for his senior teammate, it could be Balotelli's presence which pushes Niang into Chelsea's arms this summer.