Aims: To identify the 1) prevalence of meeting the endpoints of ‘stable periodontitis’ (probing pocket depth (PPD) ≤4mm, bleeding on probing (BoP) <10%, no BoP at 4mm sites), ‘endpoints of therapy’ (no PPD >4mm with BoP, no PPD ≥6mm), ‘controlled periodontitis’ (≤4 sites with PPD ≥5mm), ‘PPD <5mm’ and ‘PPD <6mm’ at the start of supportive periodontal care (SPC) and 2) identify any variables which may affect endpoint achievement. Materials and Methods: This service evaluation evaluated consecutive patients entering SPC between 2020 and 2022 at Guy’s Hospital, London. Data collected included periodontal parameters (PPD, FMPS, FMBS) at baseline and their final active periodontal treatment (APT) visit and were analysed using SPSS 29.0 for descriptive and quantitative statistics. Logistic regressions utilised univariable and multivariable models to examine associations between clinical variables and outcomes. Results: This analysis included 141 subjects (mean age 45.4 ± 1.1 years; 64.5% female; 58.9% Caucasian). Most subjects had generalised Stage III/IV periodontitis (93.6%). At SPC entry, 12.77% of subjects exhibited stable periodontitis, and 48.23% met the endpoints of therapy. Relative percentage changes from baseline to end of APT in achieving stable periodontitis/ endpoints of therapy in teeth were as follows: all teeth (66.25% / 90.97%), anteriors (81.75% / 97.12%), premolars (74.20% / 94.69%), and molars (52.40% / 86.01%). Regression analyses confirmed associations between age and disease extent with endpoints but explained limited variance (R² ≤ 0.267). Conclusions: Periodontal treatment significantly improved clinical parameters at site and tooth-level, though molars demonstrated lower endpoint achievement highlighting localised treatment challenges. Age and disease extent influenced outcomes, but predictor variables explained only modest variance, highlighting the complexity of achieving therapeutic success in periodontitis management.