Past studies described occasional patients with myeloid neoplasms (MN) and coexistent large granular lymphocytic leukemia (LGLL) or T-cell clonopathy of unknown significance (TCUS), which may represent expansion of myeloid clonal hematopoiesis (CH) as triggers or targets of clonal cytotoxic T cell reactions. We retrospectively analyzed 349 LGLL/TCUS patients, 672 MN patients, and 1443 CH individuals to establish the incidence, genetic landscape, and clinical phenotypes of CH in LGLL. We identified 8% of cases overlapping with MN, while CH was found in an additional 19% of cases (CH + /LGLL) of which TET2 (23%) and DNMT3A (14%) were the most common. In MN cohort, 3% of cases showed coexistent LGLL. The incidence of CH in LGLL was exceedingly higher than age-matched CH controls (P < 0.0001). By multivariate analysis, the presence of CH in LGLL (P = 0.026) was an independent risk factor for cytopenia in addition to older age (P = 0.003), splenomegaly (P = 0.015) and STAT3/5B mutations (P = 0.001). CH + /LGLL cases also showed a higher progression rate to MN than CH-/LGLL (10% vs. 2% at 5 years; P = 0.02). A close relationship between CH and LGLL suggests that cytopenia in LGLL may be not only related to LGLL but be also secondary to coexisting clonal cytopenia of unclear significance.