Responsive school mental health services require proactive family engagement at the outset of prevention and early intervention, particularly across key transition periods such as kindergarten. Universal screening for mental health is one important process within a school mental health framework to proactively engage families in a collaborative data-based decision-making and problem-solving process. In the current study, we tested the psychometric properties of the Positive Family Support–Strengths and Needs Assessment (PFS–SaNA), a brief parent–report screener of children’s social, emotional, and behavioral functioning. Participants included 365 parents/caregivers who were assessed across kindergarten to second grade. We assessed the construct and predictive validity of the PFS–SaNA, tested the longitudinal invariance of the measure across kindergarten to second grade, and established the reliability of the measure. We replicated the unidimensional structure of the PFS–SaNA identified in prior literature (Moore et al. 2016) and found that this single construct is invariant from kindergarten to second grade. Predictive and concurrent validity analyses with convergent behavioral measures show the PFS–SaNA has adequate predictive and diagnostic capability. Moreover, we found the single factor to have adequate reliability. Implications of these findings for comprehensive mental health support that includes family collaboration in the transition to schooling are discussed.