Increasingly, graduate student instructors (GSIs) teach online without prior formal training on how to design online assessments of learning and participation. We present findings from a collective case study of seven GSIs at a university in the United States of America to describe how these novice online instructors learned to enact assessment strategies in an online classroom. Findings reveal that the GSIs were influenced by professional learning networks such as peer communities of practice, the academic Twitter community, student feedback, and faculty gatekeeping of institutional resources and policies. With limited institutional and departmental support, the GSIs struggled to transform their face-to-face assessments to leverage technology but leaned on their professional judgment to prioritize traditional discussion and participation structures. Findings inform our understanding of how universities might prioritize professional learning networks on online teaching at differing institutional levels to increase GSIs' access to online teaching information and knowledgeable role models.