Inactivation of the Caenorhabditis elegans gene clk-1, which is required for ubiquinone biosynthesis, increases lifespan by an insulin signaling-independent mechanism. We find that homozygous inactivation of mclk1, the mouse ortholog of clk-1, yields ES cells that are protected from oxidative stress and damage to DNA. Moreover, in the livers of old mclk1^+/- mice, hepatocytes that have lost mclk1 expression by loss of heterozygosity undergo clonal expansion, suggesting that their resistance to stress allows them to outcompete cells that still express the gene. mclk1^+/- mice, whose growth and fertility are normal, also display a substantial increase in lifespan in each of three different genetic backgrounds. These observations indicate that the distinct mechanism by which clk-1/mclk1 affects lifespan is evolutionarily conserved from nematodes to mammals and is not tied to a particular anatomy or physiology.