After recovering from COVID-19, patients may experience persistent symptoms, known as post-COVID-19 syndrome or long COVID, which include a range of continuing health problems. This research explores the prevalence, associated factors, and overall health outcomes of long COVID during a period of extensive vaccination and antiviral treatment availability in Thailand. This observational study involved 390 adult patients with COVID-19 between January and March 2022. Beginning three months after their diagnosis, these patients were interviewed via telephone every three months for a period of one year. The data collection process included gathering demographic information and administering a standardized questionnaire that addressed the patients' physical condition following COVID-19, their mental health, sleep disturbances, and overall quality of life. The cohort consisted of 390 participants, with an average age of 31.8 ± 13.6. Among them, 96.7% ( = 377) were vaccinated, and 98.2% ( = 383) underwent antiviral treatment. Long-COVID prevalence was observed at 77.7%, with the most frequently reported symptoms being fatigue (64.1%) and cough (43.9%). Regarding mental health, depression was reported by 8.2% of the participants, anxiety by 4.1%, and poor sleep quality by 33.3%. Advanced statistical analysis using multivariable logistic regression showed significant links between long-COVID symptoms and patients aged below 60 ( = 0.042), as well as the initial symptom of cough ( = 0.045). In the subset of long-COVID sufferers, there was a notable correlation in females with symptoms such as headaches ( = 0.001), dizziness ( = 0.007), and brain fog ( = 0.013). Despite the extensive distribution of vaccines and antiviral therapies, the prevalence of long COVID remains high, being associated particularly with individuals under 60 and those exhibiting a cough as an early symptom. The study further reveals that mental health issues related to long COVID are profound, going beyond the scope of physical symptomatology.