While neuronal activity is important for synapse refinement and maintenance, its contribution to synapse formation and dendritic arborization remains unclear. Our previous work in dissociated hippocampal neurons identified a timeframe prior to the establishment of synapses during which spontaneous glutamate release facilitates the branching of dissociated hippocampal neurons by targeting NMDA receptors (NMDARs). We now demonstrate the role of NMDARs in synapse formation and dendritic maturation in hippocampal CA1 pyramidal neurons in cultured slices and in vivo. To test whether NMDARs guide synapse formation during development, we pharmacologically blocked NMDARs at different timings in organotypic slices. Blocking NMDARs during late embryonic development reduced Schaffer collateral synapse densities in CA1, while no effects were seen following postnatal perturbation. In order to sparsely block NMDAR function during early development in vivo, we performed in utero injections of AAVs expressing Cre and a flexed reporter in transgenic mice with a conditional deletion of the obligatory GluN1 subunit. We confirmed complete loss of NMDAR function in CA1 pyramidal neurons using whole-cell patch-clamp electrophysiology. Similar to the results in organotypic slices, we observed effects on excitatory synapse densities and dendritic morphology. Using timelapse in vivo structural imaging we noticed that cells expressing NMDARs exhibited more dynamic dendritic arbors. In vivo functional imaging further revealed that NMDARs are frequently activated and give rise to minute-long dendritic calcium transients which correlate with the emergence of branching filopodia. Overall, our results shed light on the developmental role of NMDARs during synaptogenesis and dendritic arborization.