Across the American republic, pundits, politicians, and social scientists alike have studied and speculated about the rising tide of partisan divisiveness threatening to inundate the political mooring of American society. Known as partisan and affective polarization, these phenomena refer to the increased import and sorting effect of partisan identities, such as Democrat or Republican, as exacerbated by a salient dichotomy of partisan attitudes inclusive of both animosity and aversion toward opposing partisans, synonymously denoted partisan animus or affective polarization, as well as positive sentiment and attraction toward copartisans, known as partisan homophily. Although scholars often fixate on the external political ramifications of corporations, too often we fail to ask how the manifestation of increasing partisanship in society affects the careers, behavior, and structure within firms. Existing research demonstrates the very social nature of informal practices in corporate organizations, wherein social and cultural fit can shape individual career trajectories. We also find a number of efficiencies garnered from homogenous teams and organizations, not to mention the categorical trust and affinity found amongst similar others. Although organizational research illustrates the potential downfalls of diversity for teams, groups, and corporate boards, we also witness that in some cases, organizations might generate innovation and creativity by drawing from diverse perspectives. Legal and regulatory incentives also exist to preempt discrimination, suggesting that firms might make best-faith efforts to likewise avoid discrimination. In these ways, firms seemingly have several incentives to prefer partisan homogeneity, but at the same time face arguable performance, legal, and regulatory rationales to preempt partisan bias and promote diversity. Taken together, we can formalize this puzzle by asking: To what degree has political partisanship emerged as a structuring mechanism in the American corporation, or more simply, what effect does partisanship have on careers within firms? To evaluate this central theoretical question, I first evaluate the change in partisan polarization in firms from 1980 to 2018, finding that particularly since 2012, we have seen a rise in partisan polarization, as evidenced by the increased prevalence of public partisan attachment in firms. This includes the emergence of polarized Democratic and polarized Republican firms, which have high partisan homogeneity across multiple levels of the occupational hierarchy. In other words, entry-level employees, managers, and executives are increasingly similar in their partisan identity within firms. Second, through both a computational field experiment and a longitudinal analysis of corporate board appointments, I find results consistent with affective polarization and partisan homophily. Job applicants whose partisanship opposed that of a firm were less likely to receive callbacks compared to applicants whose partisanship aligned with the firm. Similarly, corporate boards were also significantly more likely to appoint copartisan versus opposing partisan board members. These results substantiate my argument that the societal rise of partisanship not only permeates the corporation, but indeed fundamentally reshapes its internal organizational structure, affecting not only the partisan balance of a firm, but also redefining exactly who is welcome to join a given firm, proceed therein, and rise through the ranks as a valued employee.