Background: The management of degenerative hip diseases in young patients remains a challenge. Despite the improvement of hip-preserving procedures, total hip arthroplasty (THA) may be required in some instances. In addition, young patients undergoing THA have high expectations concerning their postoperative level of activity. Purpose: (1) to define the sports activity level and the return to sports after THA, (2) to describe the modification or initiation of new sports disciplines, and (3) to report the clinically meaningful outcomes after THA in patients younger than 40 years. Methods: A total of 36 patients (40 hips) were prospectively analyzed at a midterm follow-up of 3.9 years. The modified Harris Hip Score (mHHS); the Visual Analog Scale (VAS) for pain; the University of California, Los Angeles (UCLA) activity scale; and sports and recreational activity levels were assessed via questionnaire. The minimal clinically important difference (MCID) was determined by calculating half of the standard deviation, and the substantial clinical benefit (SCB) as well as patient acceptable symptomatic state (PASS), were calculated by the anchor method for the mHHS. Results: At the final follow-up, there was a significant improvement in mHHS (34.1 to 92.6; p < 0.0001), UCLA (3.2 to 7.6; p < 0.0001), and VAS for pain (8 to 1; p < 0.0001). More patients were active in sports at follow-up than before surgery (44% to 92%, p < 0.0001). In addition, the duration and frequency of sports activities showed a significant increase (p < 0.0001). The MCID, SCB and PASS for mHHS were 89% and 58%, respectively. No revision surgery had to be performed. Conclusion: This study showed that a large proportion of patients under 40 years of age who underwent THA increased their physical activity. Eighty-six percent of the patients were highly active, with a UCLA score ≥ 7. Furthermore, the reported MCID, SCB, and PASS for mHHS were achieved by more than 80% of patients.