The 6-3 ruling shed light on how the high court views the roles of Crown prosecutors.
A minority of three judges found that the law was not, in fact, unconstitutional because it still gave some discretion to prosecutors, allowing them to decide whether to proceed by summary conviction or by indictment.
The summary conviction option allows for a one-year sentence, while only the indictable offence carries the mandatory minimum of three years for a first conviction.
Justice Michael Moldaver, who wrote for the minority, argued in favour of maintaining the law.
Chief Justice Beverley McLachlin, writing for the majority, strongly rebutted that argument.
Here is what they said:
"The least blameworthy conduct — licensing-type cases — is meant to be prosecuted summarily, thereby avoiding the application of the mandatory minimum . . .
"Parliament's intention to divert the least serious cases into summary proceedings is critical to assessing the constitutionality . . .
"By creating a 'safety valve' to shield licensing-type cases from the reach of the mandatory minimum, Parliament has effectively conceded the existence of reasonably foreseeable cases in which a mandatory minimum would be grossly disproportionate."
"The protection it offers against grossly disproportionate punishment is illusory: in practice it would create a situation where the exercise of the prosecutor's discretion is effectively immune from meaningful review . . .
"It gives prosecutors a trump card in plea negotiations, which leads to an unfair power imbalance with the accused and creates an almost irresistible incentive for the accused to plead to a lesser sentence in order to avoid the prospect of a lengthy mandatory minimum term of imprisonment . . .
"We cannot ignore the increased possibility that wrongful convictions could occur under such conditions."