Background: Identifying factors that determine the frequency of latently infected CD4+ T cells on antiretroviral therapy (ART) may inform strategies for human immunodeficiency virus (HIV) cure. We investigated the role of CD4+ count at ART initiation for HIV persistence on ART. Methods: Among participants of the Strategic Timing of Antiretroviral Treatment Study, we enrolled people with HIV (PWH) who initiated ART with CD4+ T-cell counts of 500–599, 600–799, or ≥ 800 cells/mm3. After 36–44 months on ART, the levels of total HIV-DNA, cell-associated unspliced HIV-RNA (CA-US HIV-RNA), and two-long terminal repeat HIV-DNA in CD4+ T cells were quantified and plasma HIV-RNA was measured by single-copy assay. We measured T-cell expression of Human Leucocyte Antigen-DR Isotype (HLA-DR), programmed death-1, and phosphorylated signal transducer and activator of transcription-5 (pSTAT5). Virological and immunological measures were compared across CD4+ strata. Results: We enrolled 146 PWH, 36 in the 500–599, 60 in the 600–799, and 50 in the ≥ 800 CD4 strata. After 36–44 months of ART, total HIV-DNA, plasma HIV-RNA, and HLA-DR expression were significantly lower in PWH with CD4+ T-cell count ≥ 800 cells/mm3 at ART initiation compared with 600–799 or 500–599 cells/mm3. The median level of HIV-DNA after 36–44 months of ART was lower by 75% in participants initiating ART with ≥ 800 vs 500–599 cells/mm3 (median [interquartile range]: 16.3 [7.0–117.6] vs 68.4 [13.7–213.1] copies/million cells, respectively). Higher pSTAT5 expression significantly correlated with lower levels of HIV-DNA and CA-US HIV-RNA. Virological measures were significantly lower in females. Conclusions: Initiating ART with a CD4+ count ≥ 800 cells/mm3 compared with 600–799 or 500–599 cells/mm3 was associated with achieving a substantially smaller HIV reservoir on ART.