Objective: The study objective was to assess participant weight change for the English National Health Service (NHS) Digital Weight Management Programme, the first such digital intervention to achieve population coverage. Methods: A service evaluation was used to assess intervention effectiveness for adults with obesity and a diagnosis of hypertension and/or diabetes, between April 2021 and March 2022, using prospectively collected, national service–level data in England. Results: Of the 63,937 referrals made from general practices, within the time period, 31,861 (50%) chose to take up the 12-week Programme. There were 31,718 participants who had time to finish the Programme; of those, 14,268 completed the Programme (defined as attending ≥60%), a 45% completion rate. The mean weight change for those who had time to finish the Programme was −2.2 kg (95% CI: −2.25 to −2.16), for those who completed it was −3.9 kg (95% CI: −3.99 to −3.84), and for those who had time to finish the Programme but did not complete it was −0.74 kg (95% CI: −0.79 to −0.70). Conclusions: The NHS Digital Weight Management Programme is effective at achieving clinically meaningful weight loss. The outcomes compare favorably to web-based weight management interventions tested in randomized trials and those delivered as face-to-face interventions, and results suggest that the approach may, with increased participation, bring population-level benefits.