Framing professional learning as a social practice underscores the interplay between subjective meaning systems and objective conditions of the social space where learning occurs. Drawing on Pierre Bourdieu's theory of social practice, and methodologically guided by critical realism, the paper identifies what constitutes effective teacher professional learning through mentoring. The empirical focus of the paper is an Early Childhood teacher mentoring programme in the state of Victoria, Australia. The findings show that: (a) relevant circumstances that make teacher professional learning a necessity are associated with individual aspirations and systemic requirements; (b) collegial relationships and critical deliberation constitute meaningful learning experiences; and (c) teacher learning takes place in the domains of professional dispositions, pedagogical knowledge and social capital. The implication is that in designing professional development programmes such as mentoring, it is important to take teachers' contexts of practice into account; conceptualise learning as a socially situated practice; and recognise the value of teachers' lived experiences as a locus of deliberation and learning.