In this article I argue that in their current genealogical and philosophical configuration, qualitative longitudinal research (QLR) practices -- and a wider regime of knowledge, ethical, moral, legal, technological, political and economic practices with which they are entangled -- embed and enact representational assumptions in which the realities being investigated -- time, change and continuity; the past, present and future -- are taken as ontologically given and independent of these QLR (and wider) practices. My approach is to conceptualize QLR practices along nonrepresentational lines, through a philosophical framework that is able to materialize the constitutive effects of QLR (and wider) practices on the objects of study and knowledges produced. For this, I turn to Karen Barad's posthumanist performative metaphysics -- 'agential realism' -- a framework that embodies and enacts a non-classical ontology in which entities are seen as constituted through material-discursive practices. On this account, QLR (and wider) practices are understood as an ineliminable and constitutive part of the realities they help bring into being.