The Supreme Court of Canada (SCC) makes constitutional law decisions with wide-ranging policy impacts. These rulings are made in the context of legal submissions by parties and various third-party interveners, including government and interest group actors. Yet, little scholarly attention has been paid to how legal and political mobilization by actors or groups may shape both the SCC’s rulings and legislatures’ subsequent policy-making decisions. To this end, this doctoral thesis studies 71 constitutional law cases decided by the SCC from 2009-2019 with 581 interventions by 256 unique interveners. It tracks success rates of litigants and interveners in having their policy positions (concerning the constitutionality of the laws) adopted by the SCC. It finds that there is a clear group of most frequent and most frequently successful interveners, including attorneys general, civil liberties associations, lawyers’ groups, and legal clinics. The research also demonstrates correlations between interventions (by top repeat interveners, majorities of government interveners, or majorities of all interveners) and the Court’s policy positions. Lastly, the research assesses existing explanations of interventions to build an explanation consistent with the constitutional cases studied. The thesis also examines legislative policy-making replies to the SCC’s decisions, and the role of interest group mobilization therein. For example, in cases with Charter infringements, legislative responses almost always complied with the judicial decisions; in rare instances of legislative avoidance or non-compliance, replies were consistent with third-party interventions and counter-mobilization. In Charter cases in which the laws were upheld, judicial decisions tended to solidify the policy status quo, though some exceptions include legislative non-compliance and reversal, a test of constitutional limits, an instance of ‘losers’ winning, and codification of the judicial ruling. Only one of the non-Charter cases in which the law was upheld was followed by policy change, which tested the constitutional limits. Most of the legislative replies to decisions which upheld the laws were formulated in the seeming absence of mobilization to the contrary, though one exception was mobilization by law enforcement and others to restrict access to supervised consumption sites following PHS. Finally, in non-Charter cases where the laws were deemed unconstitutional, a minority of cases involved policy change (generally) in compliance with the judicial decisions.