A dysfunctional striatum is thought to contribute to neurodevelopmental disorders such as ADHD, Tourettes syndrome and OCD. Insight into these disorders is reliant on an understanding of the normal development of the striatal cellular and circuit properties. Here we combined whole-cell patch-clamp electrophysiology and anatomical reconstructions of D1 and D2 striatal projection neurons (SPNs) in brain slices to characterize the development of the electrophysiological and morphological properties as well as their long-range and local inputs during the first three postnatal weeks. Overall, we find that many properties develop in parallel but we make several key observations. Firstly, that the electrophysiological properties of young D1 SPNs are more mature and that distinctions between D1 and D2 SPNs become apparent in the second postnatal week. Secondly, that dendrites and spines as well as excitatory inputs from cortex develop in parallel with cortical inputs exhibiting a prolonged period of maturation involving changes in postsynaptic glutamate receptors. Lastly, that initial local connections between striatal SPNs consist of gap junctions, which are gradually replaced by inhibitory synaptic connections. Interestingly, relative biases in inhibitory synaptic connectivity seen between SPNs in adulthood, such as a high connectivity between D2 SPNs, are already evident in the second postnatal week. Combined, these results provide an experimental framework for future investigations of striatal neurodevelopmental disorders and show that many of the cellular and circuit properties are established in the first and second postnatal weeks suggesting intrinsic programs guide their development.