The occurrence of mycotoxins causes substantial reductions in maize (Zea mays L.) grain quality worldwide. The predominant mycotoxins found in maize grains are aflatoxins (AFB1, AFB2, AFG1 and AFG2), fumonisins (FB1 and FB2), deoxynivalenol (DON), toxin T2 (T2), ocratoxin A (OTA) and zearalenone (ZEA). In Europe, the predominant mycotoxins originated by field contaminations are produced by mycotoxigenic fungi mainly belonging to the Fusarium genus. Accurate fungal identifications, mycotoxin detection and occurrence estimations are important, however, in Portugal, the knowledge about the incidence of mycotoxin types in pre-harvested maize grains and during its storage is still limited. The incidence of mycotoxins in maize grains is substantially influenced by the agricultural practices where the harvesting time is of upmost importance to minimise the risk of accumulation. The main objective of this work is to evaluate, for the first time, the incidence of different types of mycotoxins on maize grains harvested on three farmers located at the center of Portugal. For this purpose, grains from three harvesting dates and three maturation stages were analyzed by LC/MS/MS. It has been shown that fumonisins (FB1 and FB2) were the main mycotoxins contaminant in the samples analysed. No other mycotoxins were detected. Among fumonisins, FB1 was the most predominant. Additionally, our data also indicates that the risk of contamination by FB1 and FB2 increases with late harvestings. Therefore, a good knowledge of climatic conditions may lead to the establishment of adequate field practices, in particular, the forecast of an early harvesting time.