The fruit and vegetable food industries face a major problem: the generation of large amounts of waste, which can translate into a high amount of food by-products. Fennel is a very aromatic plant which has a characteristic anise flavor. Fennel leaves are rich in bioactive compounds such as phenolic acids, tocopherols, flavonoids and chlorophyll. The aims of this investigation were to study the effect of convective dehydration (50, 60 and 70°C) on total phenolic (Folin–Ciocalteu), flavonoid and chlorophyll contents (spectrophotometric methods). Further aims were to assess the drying kinetics of fennel leaves by mathematical modelling and to determine their functional properties (antioxidant and antidiabetic capacity) related to the aforementioned bioactive compounds in order to add value to and revalorize fennel leaves as natural sources of functional ingredients. As expected, the drying time required to reach a similar moisture content (15%) decreased as the drying temperature increased. In particular, the Midilli–Kucuk model provided an excellent fit to all the experimental drying curves. Drying temperature had a significant effect on phenol and flavonoid contents. The chlorophyll content in dried samples increased compared to the control, but as the drying temperature increased, the chlorophyll content decreased. Antioxidant capacity decreased with drying temperature, but at 70ºC, it remained higher than at 50 and 60ºC. The results of this investigation indicated that the extract with the highest antioxidant activity also showed the highest α-glucosidase inhibitory activity, meaning that antioxidant activity could influence the antidiabetic behavior of the extracts exerted by the bioactive compounds. In conclusion, the leaves of fennel, which are discarded nowadays, can be an excellent source of bioactive compounds that could be applied as ingredients in the design of new functional and healthy food products.