Improper femoral component size remains a source of multiple postoperative complications following total knee arthroplasty (TKA). However, the use of a flexible intramedullary (IM) rod may help optimize femoral component size and therefore improve outcomes. The purpose of this study was to assess (1) patient-reported functional outcomes, (2) overall quality of life, and (3) changes in implant sizing associated with total knee arthroplasties performed with a flexible IM rod compared to a conventional, rigid rod. We reviewed 277 patients who had surgery using the rigid rod and 364 using the flexible rod to determine the tendency of each rod for selecting particular component sizes. Additionally, 100 patients were prospectively randomized (1:1) to the flexible or the conventional rigid IM rod cohorts. Outcomes were assessed using Knee Society scores (KSSs), SF-36 physical scores, and SF-36 mental scores preoperatively and at 6 weeks, 3 months, 1 year, and 2 years postoperatively. The retrospective arm of the study showed that the flexible IM rod cohort tends to have smaller component sizes than their conventional counterparts. In the prospective phase of the study, the increase in clinical KSSs from preoperative levels was better in the flexible rod cohort (160% vs. 143% increases, respectively). The functional KSSs had slightly higher increases in the flexible rod cohort from their preoperative levels (68% vs. 62% increases, respectively). With both clinical and functional KSSs, the flexible rod cohort had a higher score than the rigid rod cohort at all follow-up points. There was better postoperative range of motion (ROM) in the flexible rod cohort (28% vs. 22% increases, respectively). The SF-36 physical scores were slightly different, with the flexible IM rod cohort having a more marked improvement in scores (64% vs. 46% increases, respectively). The SF-36 mental score had a slightly better improvement at latest follow-up in the flexible IM rod cohort (12% vs. 6% increases, respectively). Those patients who underwent TKA using a flexible IM rod had better improvements in their patient-reported outcomes and decreased risk of oversizing the femoral component. The use of such a rod is not detrimental to outcomes and may have a positive impact on outcomes. Future studies should focus on alignment and long-term outcomes associated with the use of a flexible rod.