Low investments in sustainable land management (SLM) limit agricultural production in the East African Highlands, leading to increased soil erosion, low productivity of land and food insecurity. Recent studies in the region show that different socio-economic factors influence SLM investments by farmers, but knowledge on which of these are most influential and how to foster SLM is often lacking. This research fills this gap for the West Usambara Highlands, and determines key socio-economic factors influencing investments in SLM. Data for this research was collected from 196 randomly selected farm households in the Lushoto district, and Factor Analysis was used to reveal patterns of correlation between SLM investments and different household characteristics. Among the eight socioeconomic factors generated by Factor Analysis, four key factors were found to significantly influence SLM investments in upland (mainly rain-fed) agricultural fields: (1) the farmer’s access to support services; (2) the farmer’s experience with collaborative land management; (3) the household’s income from crop sales; and (4) the farmer’s forward driven attitude. The study concludes that a strategy to foster and scale-up SLM in the West Usambara Highlands should therefore improve the access of farmers to support services (especially micro-credits), trigger collaborative efforts in land management (e.g., farmer-to-farmer training) and promote investments in the rural area to create an enabling environment for SLM. A more active role of the Government, together with public and private sector partners, is therefore crucial.