Most mutations in coding regions of the genome are deleterious, causing selection to favor mechanisms that minimize the mutational load over time [1–5]. DNA replication during cell division is a major source of new mutations. It is therefore important to limit the number of cell divisions between generations, particularly for large and long-lived organisms [6–9]. The germline cells of animals and the slowly dividing cells in plant meristems are adaptations to control the number of mutations that accumulate over generations [9–11]. Fungi lack a separated germline while harboring species with very large and long-lived individuals that appear to maintain highly stable genomes within their mycelia [8,12,13]. Here, we studied genomic mutation accumulation in the fairy-ring mushroom Marasmius oreades. We generated a chromosome-level genome assembly using a combination of cutting-edge DNA sequencing technologies, and re-sequenced 40 samples originating from six individuals of this fungus. The low number of mutations recovered in the sequencing data suggests the presence of an unknown mechanism that works to maintain extraordinary genome integrity over vegetative growth in M. oreades. The highly structured growth pattern of M. oreades allowed us to estimate the number of cell divisions leading up to each sample [14,15], and from this data we infer an incredibly low per mitosis mutation rate (3.8 × 10-12 mutations per site and cell division) as one of several possible explanations for the low number of identified mutations.