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Cardiac cell therapy using cardiac mesenchymal cells (CMC) significantly reduces ventricular dysfunction in patients with ischemic cardiomyopathy. Despite the improvement in function, a modest number of CMCs survive in the heart post-transplantation. In this study, we sought to improve the survival and retention of transplanted CMCs to prolong the therapeutic benefits afforded by cardiac cell therapy. To do this, we targeted the enzyme telomerase (TERT), known to be active in some highly proliferating cells (e.g. germ, stem). TERT is responsible for preventing telomere attrition, thereby allowing continued proliferation. TERT has also been shown to be protective, improve cell migration and stimulate angiogenesis. These actions make the expression of TERT an ideal target. In this study, we overexpressed TERT in CMCs using a lentiviral vector. While TERT overexpression immortalized other cell types, in our hands TERT overexpression did not result in increasing CMC lifespan. Also, overexpression did not improve migration or oxidative stress resistance in CMCs. While TERT is commonly believed to be silent in somatic cells, it has been shown to be detected in the heart. So, we also attempted to exhaustively identify endogenous TERT expression in CMCs, but expression in CMCs could not be detected. To go a step further, we then attempted to identify TERT expression in a highly proliferative subpopulation of clonogenic CMCs. Clonogenicity is a known characteristic of TERT expressing cells, but we were unable to detect TERT in clonal CMCs. Although we did not find TERT expression and overexpression was fruitless, we did identify more resilient, proliferative CMCs using the cloning technique. We successfully identified CMC clones with differential stress resistance, paracrine stimulation and growth rate that appear morphologically and genetically distinct. Furthermore, we found that transplanting clonogenic CMCs with reduced stress resistance does not ameliorate cardiac function in chronically infarcted rat hearts. However, transplanting unsorted CMCs caused a modest, but significant, improvement of cardiac function. These results suggest that subpopulations exist within the unsorted CMCs that provide no therapeutic benefit. It also suggests a correlation between in vitro stress resistance and reduction in ventricular impairment.