Introduction Most centers in low- to mid-income countries (LMICs) lack facilities for a comprehensive instrumented gait analysis (IGA) which is often considered the preferred method for assessment of the functional results of surgery in children with spastic diplegia. We aimed to study if there were any changes in the Gross Motor Function Classification System (GMFCS) levels and Functional Mobility Scale (FMS) scores after surgery and whether they can be used as an indirect indicator of change in the functional status of a child. Methods This prospective study was conducted at the Pediatric Orthopedic unit of a teaching hospital on spastic diplegic children requiring surgical intervention. GMFCS levels and FMS scores were recorded before the surgery and at each follow-up visit, with the latest record being two years post-surgery. The change in the scores was indicated as an improvement, deterioration, or no change from the baseline and compared to the score of the preceding visit. In addition, it was examined whether the age at surgery had any effect on the temporal change in the scores. Results A total of 25 children were included for analysis after excluding those who failed to fulfill the predefined inclusion and exclusion criteria. Both the GMFCS levels and FMS scores improved from the third month to one-year post-surgery, after which a few patients had a worsening of their scores at the two years follow-up visit. The age at which surgery was performed had no significant effect on the pattern of change in the scores. Most children sought consultations with the physical therapy department only when they visited the surgical team for follow-up. Conclusion This study shows that surgical interventions do improve the functional outcomes in children with spastic CP when assessed using FMS scores while maintaining an undeteriorated GMFCS level in most children. While a peak improvement can be expected one year after surgery in most patients, possible of worsening from baseline scores do exist, and the parents must be informed of the same. Any decision for surgery must involve the parents, and the usefulness of postoperative physical therapy must be impressed upon them before the surgery and during each follow-up visit too.