Fruits and aerial parts of lingonberry could be better developed as dietary supplements if the composition in bioactive phenolic compounds and the best period for collection were known. UPLC/MS analysis revealed the predominant presence of arbutin in leaf and that of flavanols in stems harvested in May, July and September. Anthocyanins, flavanols and benzoic acid derivatives were equally present in fruits. Stem and leaf are highly homologous with (+)-catechin, A- and B-type dimers/trimers, and two quercetin glycosides as major contributors. No or only weak seasonal variations were highlighted for all phenolic classes. Additionally, flavanol oligomers showed a lower mDP for fruit (3–4) than for stem and leaf (4–6). The rate of A-type linkage was 3–5% with A-type subunits in extension mainly. Finally, the content in phenolic compounds (UPLC) correlated well with TPC and the DPPH radical scavenging activity although leaf and stem constituents reacted differently in both antioxidant tests.