Alzheimer's disease (AD) is the most common cause of dementia in adults 65 years and older. Unchecked, the disease will reach epidemic proportions in the United States and worldwide by 2050, and presently, there is no intervention that has shown a clear effect on AD progression. Over the past several years, there has been increasing interest in re-purposing the use of lithium for diseases involving neurodegeneration. Lithium treatment has been associated with neurogenesis in the hippocampus, up-regulation of important neurotrophic factors such as B-cell lymphoma 2 (Bcl-2) and brain-derived neurotrophic factor (BDNF), and inhibition of glycogen synthase kinase 3 (GSK-3) isoforms α and β. In particular, GSK-3α interacts with gamma-secretase playing a critical role in the conversion of amyloid precursor protein (APP) to amyloid-beta (Aβ); lithium has been shown to reduce Aβ production and memory deficits in AD transgenic mouse models. GSK-3β phosphorylates tau, a critical step in the formation of neurofibrillary tangles, and lithium has been shown to reduce tau phosphorylation in vivo and in vitro. That lithium may alter the AD trajectory is supported by numerous observational reports showing delay of dementia onset in those treated with it. However, the results of the few human lithium trials conducted have been mixed. Additional research is needed to determine whether lithium has a role as an anti-dementia agent. In contrast to previous studies, we will implement an Randomized Controlled Trial (RCT) with a more integrative, comprehensive approach than done before involving state-of-the-art ultra-high field (7T) human Magnetic Resonance Imaging (MRI), neurocognitive assessment, and blood- and Cerebrospinal Fluid (CSF)- based biomarker measurement to investigate the role of lithium as an anti-dementia agent. The specific aim of this pilot-feasibility study is to examine the potential disease modifying properties of lithium in individuals with mild cognitive impairment (MCI) in delaying conversion to dementia. The study will enroll and randomly assign 80 individuals 60 years and older with MCI (amnestic type, single or multiple domain) to take lithium, titrated to a maximally tolerated blood level (0.5 to 0.8 meq/L), or placebo for two years to assess lithium's effects on preserving cognition and delaying conversion to dementia. Participants will receive annual neurocognitive assessment, blood- and CSF-based biomarker measurement, and 7T MRI of structural brain volumes (e.g., hippocampal, total cortical gray). At baseline, all subjects will undergo Positron Emission Tomography (PET) imaging for Aβ. The following hypotheses will be tested: H1: a) Participants randomized to take lithium for two years, compared to placebo, will better maintain cognitive function, primarily in memory, which b) will be associated with changes in biomarkers (e.g., GSK-3β, BDNF). H2: a) Participants randomized to take lithium, compared to placebo, will have larger hippocampal volumes and lower total gray matter thinning, which b) will be associated with changes in biomarkers and c) better cognitive function, primarily in memory. The exploratory aim examines whether lithium is related to additional markers of enhanced brain integrity (e.g., lower level of microbleeds, higher white matter integrity, better network connectivity, or decreased CSF phospho tau levels).