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This pre-feasibility study evaluates the use of residual leafy green biomass from broccoli (Brassica oleracea, var. Italica) and kale (Brassica oleracea, var. Sabellica) as feedstock for protein fractionation and potential application of the fractions in food and feed products. The protein concentration, protein recovery potential and the content of phenols and dietary fibre in these biomass sources and fractions were investigated. Field produce and side-stream analysis showed that among broccoli and kale side-streams the potentially suitable leaves for protein fractionation constitute up to 16 and 1.9 t/ha (DM content), respectively. Fractionation demonstrated that between 34–42 and 25–34 kg total protein could be extracted per t DM of broccoli and kale residue leaves, respectively. The amount of protein was generally high in green protein fraction (GPF) and the white protein concentrate (WPC) of both crops, although significantly higher in broccoli compared to kale. The recovery of bound and free phenolic compounds was up to 18% in the GPF of both crops, while only 0.4% ended up in the WPC. The economic assessment showed that the feedstock and processing costs of producing GPF and WPC, as well as of the combined protein fraction (CPF) 1.9–6.0 and 1.3–3.9 times higher than expected revenues for broccoli and kale, respectively, indicating that the production of protein fractions is not economically feasible with the current production scheme. However, potentially higher revenues may be obtained if value-added products such as fractionated phenols and dietary fibre components are also included and investigated in future production schemes. The pathway investigated, that included a direct drying and milling of leaf biomass showed a low processing cost and thereby the most favourable economic alternative, with approx. 7–30% profit for kale, while for broccoli revenues covered only 44–47% of the costs due to the extra harvest cost of the broccoli leaves.