The onset of Chronic fatigue syndrome (CFS) and Myalgic Encephalomyelitis (ME) is considered a key area of inquiry. Case criteria for ME and CFS and much of the academic literature suggest that patients typically experience one of two possible onset patterns: sudden or gradual. The current study provided an in-depth investigation of ME and CFS onset in order to provide insight into early symptoms, onset duration, and the progression of functional disability. We collected qualitative descriptive data to gain a rich description of illness onset from the patients’ point of view. Overall, qualitative findings revealed detailed descriptions of ME and CFS onset experiences. Major themes that emerged from the data included: onset/illness progression patterns, illness causes, methods of adapting and coping, hardworking and active lives prior to onset, healthy lives prior to onset, prior health problems, comorbid health conditions, emotional responses to onset, exertional effects, the illness as life limiting, stress, traumatic experiences, lack of support, support, and treatment limitations. A closer examination of the onset/illness progression patterns that emerged from the data provided evidence that individuals with ME and CFS experience complex onset patterns. Furthermore, the study findings suggest that the method of categorizing individuals into sudden versus gradual onset groups fails to capture the more nuanced and varied onset experiences. Prospective research studies that capture the onset period as it is developing could lead to improvements in the way we define and assess ME and CFS onset, and may also lead to methods for early detection, prevention, and individualized treatment approaches.