Historically, teachers’ unions have been some of the major organizational sites of social justice leadership in K-12 education (Kuehn, 2007;​ M. Murphy, 1990;​ Urban, 1982), but until the mid 1990s, the term “social justice unionism” (Peterson & Charney, 1999) had little currency in teacher union circles. The purpose of this qualitative case study was to examine the concept of social justice unionism in context. In particular, I asked how teacher union activists contributed and responded to the institutionalization of social justice in their organization. I used a critical constructionist (Ball, 1987;​ Berger & Luckmann, 1966;​ D. E. Smith, 1987) perspective to analyze 25 career history (Goodson, 1994) interviews with teachers, staff and elected officials affiliated with the British Columbia Teachers’ Federation between 1967 and 2007, and found that successive generations of union-involved activists dedicated to labour solidarity, feminism, multiculturalism, anti-colonialism and anti-homophobia used networks of like-minded colleagues to counter bureaucratic norms within their organization, the education system and society. A qualitative depiction of these changes suggests that they were layered, multi-dimensional and uneven. They played out on a contested, uphill gradient shaped, but not determined, by four factors:​ the organizational prioritization of teacher welfare over social justice;​ historically persistent micro-political struggles between two federation caucuses;​ the centralizing tendencies of union leadership in response to the provincial government’s centralization of educational authority;​ and broader ruling relations in Canadian society. Still, despite this uphill gradient, all activist networks left a durable trace on federation history. The major significance of this finding for critical theorists and social justice activists is a modestly hopeful alternative to the traditional conceptions of change embedded in organizational theory:​ revolution, evolution or despair.