The trenchless (T0) and trencher (T1) drainage installation methods are widely applied in Finland. There is an ongoing debate and a lack of science-based information about the performance differences between the methods. The objective was to assess drainage performance differences between T0 and T1 by analyzing groundwater table observations from field sections drained with the two methods. The differences were studied by using statistical analysis over a two-year period after the drainage installation. An experimental field in middle-Finland was divided into four T0 sections and four T1 sections. The groundwater level was manually measured about twice a week from seven locations in each section. Automatic recording was installed in one T0 section and one T1 section. The manual observations formed 56 time series, which were tested between the same-method plots (T0-T0 and T1-T1) and the different-method plots (T0-T1). Automatic data was used to validate the manual observations. In the T0 sections, 60–90% of the groundwater level observations were higher than those in the T1 sections. These observations had an average difference of 0.14–0.25 m. The variation in the groundwater level time series was larger between the T0 sections than between the T1 sections. Statistically significant differences between the same method field sections indicated that other factors also affected the groundwater table (soil type, etc.). However, the differences between T0 and T1 were stronger than those between the same-method sections, and the differences were clearest when the groundwater levels were above the drain depth (1.0 m). In the seasonal time series, the biggest differences were found during the autumn and winter periods. The average differences between T0 and T1 might not be significant in practice, but occasional larger (> 0.4 m) differences may have a short-term influence on field activities and crop growth.