The quality of turnip-rooted parsley (Petroselinum crispum var. tuberosum) as a seasoning ingredient depends on the content and composition of the essential oil. The content of essential oil is influenced by many factors, the main of which are genetic variations and environmental conditions. The presented work presents for the first time a comparison of such numerous cultivars of root parsley in terms of the essential oil content in both the roots and the leaves. The experiment compared the content and composition of the essential oils of fifteen parsley cultivars in two growing seasons. The essential oil was obtained by the hydrodistillation method, and the composition analysis was performed using the GC/MS technique. The essential oil content in parsley roots ranged from 0.013 to 0.045 mL 100 g−1 FW, while in leaves, it was within the range of 0.041–0.121 mL 100 g−1 FW. The leaf essential oil yield was also higher than the roots, proving that parsley leaves are a valuable spice and should not be treated as a waste product as before. It was observed in the research that the content of essential oils changed significantly due to the weather conditions in the analysed growing seasons; however, some cultivars, such as ‘Kinga’, ‘Eagle’, and ‘Berlińska PNE’, had a stable content of essential oils in the analysed period. The dominant component of the essential oils obtained from the roots of all cultivars was apiol. However, in the case of the essential oils obtained from the leaves, the main ingredients were myristicin, β-pinene, Z-falcarinol, and β-phellandrene, but their content was highly varied depending on the weather conditions in the analysed growing seasons.