Hamilton has endured a tumultous season, having been involved in disputes with stewards, his own McLaren team and other drivers.
"Lewis is not mature and does not have a head on him," Mansell told F1 Racing magazine.
"I think he needs to settle himself down because the recent episode at Monaco was unneccessary and unflattering both for himself and for Formula 1.
"I was very disappointed and he's got to remember that he has been a world champion so he should act appropriately and accordingly at all times."
The next race in the Formula 1 calendar is at Nurburgring, Germany on July 24.