Flax cultivation is influenced by geographical conditions and soil properties, affecting yield and fiber quality. This study examines the performance of two fiber flax varieties, Artemida and Hermes, in 2021–2023 in central–eastern Poland’s agroclimatic and soil conditions using a value chain approach. Field trials were conducted in soils of varying fertility under a continental climate, employing a randomized block design with four replications. Flax straw underwent dew-retting, and long fibers were extracted through laboratory scutching. Results showed significant differences between the varieties. Artemida achieved higher straw yields, particularly in moderately fertile soils, while Hermes produced a higher proportion of long fibers and adapted better to less-fertile soils. Hermes fibers were thinner and more delicate, whereas Artemida fibers were coarser and stronger. The average straw yield of the Artemida variety was higher by 1.5 t ha−1 than that of the Hermes variety. The yield and quality of fiber were determined by soil fertility and climate, and the genotype–environment interaction was of the greatest importance. These findings provide valuable insights for farmers and stakeholders in selecting suitable flax varieties for different soil and climatic conditions in central–eastern Poland. The value chain approach also supports optimizing cultivation practices and improving the economic sustainability of flax production.