The selection of the lowest instrumented vertebra (LIV) for Lenke 5 and 6 curves remains a topic of ongoing debate. Recent studies support fusion to L3 due to preservation of more motion segments below the LIV. Evidence regarding long-term L3/L4 disc outcomes following fusion to L3, particularly the incidence of distal adding-on and disc degeneration is limited. A retrospective review was conducted on AIS patients with Lenke 5 and Lenke 6 curves who underwent PSF with LIV at L3. Radiographs were analysed at three intervals: preoperative, immediate postoperative, and at final follow-up. The radiographic outcome parameters were coronal balance, disc wedge (DW) angle and early L3/L4 disc degeneration which was evaluated using a validated Composite Radiographic Score (CRS). Patient-reported outcomes were assessed using the SRS-22r questionnaire. A total of 29 patients were recruited with a mean age of 15.9 ± 4.2 years; 75.9% of them were Lenke 5 and 24.1% were Lenke 6 curves. The mean duration of follow up was 75.9 ± 16.0 months. The mean postoperative distal DW angle was −1.94° which increased to −3.45° at final follow-up (p = 0.064). Adding on phenomenon was only observed in 13.8%. Coronal balance was not significantly associated with early disc degeneration or adding on phenomenon (r = −0.135, p = 0.484). At final follow-up, 27.6% remained at CRS 0, 44.8% had CRS 1 and 20.7% had CRS 2. Distal disc wedging demonstrated a weak, non-significant association with higher composite radiographic scores (r=0.254, p=0.183). In contrast, SRS-22r scores demonstrated significant improvements in function, self-image, satisfaction and total scores (3.66 ± 0.42 to 4.20 ± 0.34, p<0.001). In conclusion, LIV at L3 in Lenke 5 and 6 AIS patients provides favourable long-term clinical outcomes with a low incidence of clinically significant adding on phenomenon and low risk of L3/L4 disc degeneration.