This mixed methods study addresses learning in online chat virtual reference service at a large university library. Cultural-Historical Activity Theory (CHAT) provides the guiding principle and philosophy for the investigation, understanding, and interpretation of learning in online chat service. The research data contains a total of 2380 chat transcripts in their natural setting dated from May 1st to December 31st, 2015. Quantitative data analysis was first conducted, followed by qualitative data analysis on a corpus of 2007 chat conversations with a total of 382,793 words. The findings suggest that mediated learning in chat reference conversations is co-constructed with the technical environment that is mediated by online technology and the social environment that is mediated by social presence. The primary role of the online technology is that it enables developmental transformation of learning of the patron who benefits from both “give fish” and “teach fishing” styles of teaching.