Medication adherence is believed to be a major contributor to treatment outcomes yet studies quantifying this relationship as rare in rheumatoid arthritis (RA). To determine the association of adherence to DMARD therapy with treatment outcomes among new and existing DMARD users over 2 years. Relevant clinical parameters were obtained from a longitudinal cohort of RA patients, most of who were treated with combination therapy. Patients were classified as adherent if the proportion of days covered for each DMARD was ≥80%. Outcome measures were the change in the disease activity score in 28 joints (DAS28), simplified disease activity index (SDAI), modified health assessment questionnaires (mHAQ) and proportion of patients who achieved response criteria. An inverse propensity-score weighting method was used to estimate the association of adherence with each outcome. Of 194 patients invited, a total of 111 patients (new = 45 and existing = 66 DMARD users) met study eligibility. DMARD-naive patients demonstrated relatively higher rates of adherence compared to existing users. After controlling for confounding variables, adherence was significantly associated with reduction in DAS28 (β = −1.5, 95% CI of β = − 2.17 to −0.83, p < 0.0001), SDAI (β = −9.44, 95% CI of β = −15.53 to −3.35, p = 0.002) and mHAQ (β = −0.269, 95% CI of β, −0.462 to −0.077, p = 0.017) over 2 years among new patients and adherent patients were more likely to achieve most response criteria compared to non-adherent patients. Such associations were not replicated among existing DMARD users. Adherence to combination DMARD therapy was associated with improvements in disease activity and functional outcomes in the first 2 years of therapy.