Traumatic brain injury (TBI) affects millions of people worldwide, yet there are currently no therapeutics that address the long-term impairments that develop in a large portion of survivors. Lipid nanoparticles (LNPs) are a promising therapeutic strategy that may address the molecular basis of TBI pathophysiology. LNPs are the only non-viral gene delivery platform to achieve clinical success, but systemically administered formulations have only been established for targets in the liver. In this work, we evaluated the pharmacokinetics and activity of LNPs formulated with polyethylene glycol (PEG)-lipids of different anchor lengths when systemically administered to a mouse model of TBI. We observed an increase in LNP accumulation and activity in the injured brain hemisphere compared to the uninjured contralateral brain hemisphere. Interestingly, transgene expression mediated by LNPs was more durable in injured brain tissue compared to off-target organs when compared between 4 and 24 hours. The PEG-lipid is an important component of LNP formulation necessary for the stable formation and storage of LNPs, but the PEG-lipid structure and content also has an impact on LNP function. LNP formulations containing various ratios of PEG-lipid with C18 (DSPE-PEG) and C14 (DMG-PEG) anchors displayed similar physicochemical properties, independent of the PEG-lipid compositions. As the proportion of DSPE-PEG was increased in formulations, blood circulation times of LNPs increased and the duration of expression increased. We also evaluated diffusion of LNPs after convection enhanced delivery (CED) in healthy brains and found LNPs distributed >1 mm away from the injection site. Understanding LNP pharmacokinetics and activity in TBI models and the impact of PEG-lipid anchor length informs the design of LNP-based therapies for TBI after systemic administration.