Microsatellite (MS) mutations can potentially unravel the past of mutator phenotype tumors, with greater genetic diversity expected in older regions. Rapid clonal expansions of xenografts were characterized by relatively homogenous MS alleles, whereas greater diversity was observed in a colorectal cancer with the greatest variation in its adjacent adenoma. A subcutaneous lung cancer metastasis demonstrated diversity consistent with its one-month clinical duration and evidence of active mitosis during dormancy. The genetic legacy inherent to multistep tumorigenesis provides direct estimates of tumor ages, with up to thousands of cell divisions and high death rates necessary to yield the observed diversities. MS molecular tumor clocks have the unique potential to systematically reconstruct the early and occult evolution of individual human mutator phenotype tumors.