This article analyses why and how child support policy contributes to income inequalities as well as to cross-national variation. It uses an in-depth comparison between France and Quebec (Canada) and drives on a multi-method empirical design to assess how this policy has been designed within each welfare state regime from the 1970s to the 2020s and how family lawyers and lower court judges implement it. It shows that national conceptions of solidarity and justice differ, along with the differences between those two legal systems, leading to cross-national variations regarding child support policy. In Quebec, the neo-liberal wave and the powerful feminist movement have converged to ensure that fathers are made to face up to their financial responsibilities towards their children, which has positive effects on the wealthiest and middle-class families. In contrast, in France, the feminist movement has been less influential while the long-standing family policy has favoured state protection towards lone mothers. As a result, the financial risks involved in separation are (partially) compensated by a state system of redistribution, rather than by private remedies, which lead the French system to be more effective for lower-class families. As a result, national context still matters a lot in the part institutional arrangements and professional practices play in income inequalities after parental separations.