Background: Research shows a strong overlap between paternal criminal legal system (CLS) involvement—especially incarceration—and children's involvement with Child Protective Services (CPS) and foster care placement. Objective: To examine three explanations for the CLS-CPS overlap: (1) selection processes where a common set of factors drive CLS and CPS involvement, (2) mechanical processes, such as mandatory reporting by law enforcement to CPS, or (3) consequences that stem from paternal CLS involvement, such as reduced economic resources or increased harsh parenting. Participants and setting: The sample is 103,107 Wisconsin children and their parents followed from birth through age 12. Methods: We use discrete time hazard models to model time-varying associations between paternal CLS contact and children's CPS reports and foster care entry. Results: The odds of CPS contact and foster care entry were highest in the month of a police encounter. This was largely due to CPS reports from law enforcement, suggesting the core role of mechanical processes. Ongoing paternal incarceration did not differ from ongoing noncustodial CLS involvement in its associations with CPS reports or foster care entries. CPS reports were less likely to occur during periods of ongoing CLS involvement than in the period immediately preceding CLS contact. CPS reports from law enforcement were more likely when paternal CLS contact was due to domestic violence, drugs, or child victimization. Conclusions: The CLS-CPS overlap is not specific to paternal incarceration and may largely be driven by mandatory reporting laws following police contact.