Background/Objectives: Tissue-level markers of treatment response are lacking in severe asthma. Transthoracic ultrasound (TUS) non-invasively detects pleural-line and subpleural abnormalities that have been linked to small-airway disease, but it has not been evaluated as a marker of response to biologic therapy. The primary objective was to determine whether mean pleural-line thickness in the right and left hemithoraces changed between baseline and week 16 of biologic therapy. The secondary objectives were to describe the accompanying changes in pleural-line regularity, lung function, symptom control, and quality of life, and to examine whether the ultrasound changes are related to the clinical and functional response. Methods: In this prospective, single-center study, 41 adults with severe asthma who were starting omalizumab (n = 18), mepolizumab (n = 12), or benralizumab (n = 11) underwent standardized 14-zone TUS, spirometry, the Asthma Control Test (ACT), and the Asthma Quality of Life Questionnaire (AQLQ) at baseline and after 16 weeks. Pleural-line thickness and pleural-line regularity score were the primary TUS measures. Paired and between-group tests and Spearman rank correlations were used. Results: At week 16, 20 patients (48.8%) were complete responders. The mean ACT score increased from 13.2 ± 4.1 to 22.6 ± 2.9, the mean AQLQ total score from 3.6 ± 1.2 to 4.9 ± 1.2, FEV1 from 86.0 ± 24.8% to 99.0 ± 20.9% predicted, and FEF25–75 from 61.4 ± 35.0% to 81.2 ± 33.4% predicted (all p ≤ 0.001), whereas the blood eosinophil count fell from a median of 400 (0–1990) to 80 (0–330) cells/µL (p < 0.001). Median pleural-line thickness decreased from 1.27 to 0.94 mm in the right hemithorax and from 1.31 to 0.97 mm in the left hemithorax (both p < 0.001), and the pleural-line regularity score increased on both sides (both p < 0.001). Pleural-line thickness decreased significantly in all three biologic groups (all p ≤ 0.001), with no evidence of a differential treatment effect. Changes in pleural-line thickness did not correlate with ΔACT, ΔAQLQ, ΔFEV1, or ΔFEF25–75 (all p > 0.05). Conclusions: After 16 weeks of biologic therapy, pleural-line thickness was reduced and the pleural-line regularity score had improved on TUS. Because these pleural changes did not correlate with the symptomatic or spirometric response, TUS may capture a complementary, tissue-level dimension of the treatment effect. As the study was an uncontrolled single-arm design, the changes cannot be attributed to the biologic agent itself, and prospective controlled validation is required before TUS can be proposed as a marker of response.