The operation of small and medium sized enterprises (SMEs) has awakened the interest of experts globally due to the varied challenges from low market share, poor sales growth, and dwindling profit. These have made them less competitive, market/product failure, and sub-optimal performance, and at times closures. Hence, this study investigated the effect of managerial roles on competitive advantage of SMEs in Lagos State Nigeria. Cross sectional survey research design was adopted using structured questionnaire. The target population comprised 460 owner- managers of the selected SMEs in manufacturing, real estate, agricultural and services sector in the five main divisions of the State (Ikeja, Badagry, Ikorodu, Lagos Island, Epe). Multistage sampling technique was used. Pilot study was carried out to establish the validity and reliability of the research instrument. Cronbach’s Alpha coefficients for the constructs ranged from 0.790 and 0.946. Data were analyzed using descriptive and inferential statistics. Statistically and significant relationship was established between managerial roles and competitive advantage with adjusted R2 0.61, F-stat (4,450)=183.13, p<0.05. In addition, marketing (β = 0.529, t = 8.523, p<0.05), people management (β = 0.610, t = 13.055, p<0.05) significant affected competitive advantage while communication and negotiation exhibited insignificant effect on competitive advantage of SMEs in Lagos State. This study recommended that SMEs should actively invest in marketing and human resources to ensure competitive advantage.