Purpose The aim of this study was to determine the change in the long leg axis according to the preoperative knee phenotype using the mechanically aligned extension-first technique in total knee arthroplasty. The hypothesis of this study was that the knee phenotype would have an impact on the postoperative leg axis. Methods This was a retrospective comparative study comprising 224 whole-leg radiographs of 112 patients. The leg axes of the pre- and postoperative radiographs were measured and categorized into three preoperative limb phenotypes (based on the hip-knee-ankle angle [HKA]) according to Hirschmann et al. (varus—HKA < 178.5°, neutral—HKA 178.5°–181.5°, and valgus—HKA > 181.5°). Additionally, femoral phenotypes (based on the femoral mechanical angle [FMA], i.e., the mechanical medial distal femoral angle [mMDFA], as well as the tibial phenotypes [based on the tibial mechanical angle, i.e., the medial proximal tibial angle (MPTA)] was calculated. The change in the long leg axis was analyzed and compared with the preoperative limb phenotype. Results Significantly more patients with preoperative varus alignment shifted to neutral alignment (46.3%, n = 31) than did patients with preoperative valgus alignment (38.9%; n = 14). Moreover, 43.3% of patients (n = 29) with the varus phenotype remained in a varus alignment, compared with the 58.3% of patients with preoperative valgus phenotype (n = 21) remaining in valgus alignment. These findings were similar for both females (p < 0.001) and males (p = 0.015). Conclusion Using an extension-first mechanically aligned surgical technique, varus phenotypes predominantly result in neutral leg axes or remain varus, neutral phenotypes remain neutral, and valgus phenotypes remain valgus or change to neutral phenotypes. This study showed that preoperative knee phenotypes in valgus knees influence this technique more strongly than estimated in previous investigations, which is in line with modern alignment philosophies for TKA. Level of evidence Level IV, retrospective comparative study.