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<i>Background and Objectives</i>: Due to a lack of long-term clinical studies that would clarify whether the insertion of three mini-implants (MDIs) can be as successful treatment as the insertion of four MDIs for the retention of mandibular overdentures (ODs), this 5-year prospective cohort study was set up. <i>Materials and Methods:</i> Participants (<i>n</i> = 83) randomly received either four or three MDIs and a mandibular OD. Clinical examinations were performed at the baseline, as well as after one, three, and five years, respectively. A total of 73 participants (38 in the four-MDI and 35 in the three-MDI groups) partook in the study. The marginal bone level change, success and survival rates, and prosthodontic maintenance were assessed. <i>Results:</i> Repeated measures showed that the mean peri-implant bone loss increased progressively at a small amount over five years in both groups (four-MDI group = −0.36 ± 0.74; three-MDI group = −0.33 ± 0.27 mm; <i>p</i> < 0.05). However, an ANCOVA revealed no significant effects of the group (no significant difference between the three- and the four-MDI groups; F = 0.085; <i>p</i> = 0.771), gender (F = 0.023; <i>p</i> = 0.88), or covariate age (F = 1.95; <i>p</i> = 0.167) on the dependent variable: the 5-year MBL change. The success rate (together with successful survival) was 93.8% in the four-MDI group and 91.7% in the three-MDI group. The log-rank (Mantel–Cox) test revealed no significant differences between them (<i>X</i>² = 0.373; <i>p</i> = 0.541). <i>Conclusions:</i> In patients with narrow ridges, the insertion of three MDIs in the mandible for overdenture retention can be equally as successful as the insertion of four MDIs.