Most neuroscientists would agree that psychiatric illness is unlikely to arise from pathological changes that occur uniformly across all cells in a given brain region. Despite this fact, the majority of transcriptomic analyses of the human brain to date are conducted using macro-dissected tissue due to the difficulty of conducting single-cell level analyses on donated post-mortem brains. To address this issue statistically, we compiled a database of several thousand transcripts that were specifically-enriched in one of 10 primary brain cell types identified in published single cell type transcriptomic experiments. Using this database, we predicted the relative cell type composition for 157 human dorsolateral prefrontal cortex samples using Affymetrix microarray data collected by the Pritzker Neuropsychiatric Consortium, as well as for 841 samples spanning 160 brain regions included in an Agilent microarray dataset collected by the Allen Brain Atlas. These predictions were generated by averaging normalized expression levels across the transcripts specific to each primary cell type to create a "cell type index". Using this method, we determined that the expression of cell type specific transcripts identified by different experiments, methodologies, and species clustered into three main cell type groups: neurons, oligodendrocytes, and astrocytes/support cells. Overall, the principal components of variation in the data were largely explained by the neuron to glia ratio of the samples. When comparing across brain regions, we were able to statistically identify canonical cell type signatures - increased endothelial cells and vasculature in the choroid plexus, oligodendrocytes in the corpus callosum, astrocytes in the central glial substance, neurons and immature cells in the dentate gyrus, and oligodendrocytes and interneurons in the globus pallidus. The relative balance of these cell types was influenced by a variety of demographic, pre- and post-mortem variables. Age and prolonged hypoxia around the time of death were associated with decreased neuronal content and increased astrocytic and endothelial content in the tissue, replicating the known higher vulnerability of neurons to adverse conditions and illustrating the proliferation of vasculature in a hypoxic environment. We also found that the red blood cell content was reduced in individuals who died in a manner that involved systemic blood loss. Finally, statistically accounting for cell type improved both the sensitivity and interpretability of diagnosis effects within the data. We were able to observe a decrease in astrocytic content in subjects with Major Depressive Disorder, mirroring what had been previously observed morphometrically. By including a set of "cell type indices" in a larger model examining the relationship between gene expression and neuropsychiatric illness, we were able to successfully detect almost twice as many genes with previously-identified relationships to bipolar disorder and schizophrenia than using more traditional analysis methods.