Background/Context: Public prekindergarten (pre-K) is increasingly common in U.S. public schools. The policy decision to house pre-K classrooms in public schools places pre-K teachers in a “borderland of practice,” where the separate worlds of the early childhood and K–12 systems collide. Borderland work has implications for pre-K teachers’ job satisfaction, professional identities, and sense of belonging. Focus of Study: The purpose of this study was to understand how pre-K borderlands come to be constituted and how features of the borderland shape the lived experiences of school-based pre-K teachers. The context for the study was Michigan’s state-funded pre-K program, Great Start Readiness Program. We drew on scholarship in border studies to conceptualize the pre-K borderland as the space around the borders separating early childhood education (ECE) and K–12 systems. We sought to understand how the pre-K borderland was shaped by policies and policy actors and the implications this had for pre-K teachers’ work experiences and well-being. Research Design: This comparative case study was conducted in two Michigan counties during the 2017–2018 school year. Data included interviews and focus groups with district officials (12), interviews with pre-K teachers (28), and interviews with principals (10) and kindergarten teachers (13). Findings: Results indicate that school districts in Michigan provided pre-K through two main implementation models: elementary school and district early learning centers (ELCs). This complicates previous literature that equates school-based pre-K with the elementary model. We found that pre-K teachers in both settings faced challenges that negatively affected their work experiences and well-being. All teachers, regardless of implementation model, struggled to build professional connections with elementary school colleagues. Many also felt unsupported by administrators who lacked an understanding of pre-K. Teachers in elementary buildings had to navigate conflicting policies and building colleagues who perceived their work as “just playing.” Pre-K teachers in district ELCs faced compensation disparities that resulted in low morale and threatened to push them out of pre-K. Taken together, our findings demonstrate that neither borderland—elementary school, district ELC—was ideal, suggesting that the solution to better supporting pre-K teachers does not lie in simply altering one aspect of their work experience. Conclusions/Recommendations: School-based pre-K has multiple meanings and multiple borderlands of practice. Across these borderlands, we note the significance of classroom location decisions, teacher compensation, administrator support, and policy (in)compatibility. We advocate for (1) intentionality around the placement of pre-K classrooms within school districts, (2) compensation parity between pre-K and elementary teachers, (3) increased attention to opportunities for collaboration and professional development, and (4) fostering elementary principals’ understanding of early childhood education so they can better support pre-K teachers.