Introduction: Exercise is recognised as integral in mitigating a myriad negative consequences of cancer treatment. However, its benefit within adolescent and young adult (AYA) cancer cohorts remains relatively under researched, and caution should be taken in extrapolating outcomes from adult and paediatric populations given AYA distinctly different physiological and psychosocial contexts. This study sought to evaluate the impact of an exercise intervention on mitigating the expected decline in fitness, strength, physical functioning, and quality of life (QOL) in AYA undergoing cancer treatment. Methods: This prospective, randomised controlled trial (FiGHTINGF!T) allocated 43 participants (63% male, mean age 21.1 years) to a 10-week, multimodal, bi-weekly exercise intervention (EG) or control group (CG) undergoing usual care. Pre- and post-intervention assessments included cardiopulmonary exercise tests, one-repetition maximum (1RM) strength, functional tests, and QOL patient-reported outcome measures. Data were analysed via linear mixed models and regression. Results: While no significant group differences (p > 0.05) were observed, neither group significantly declined (p > 0.05) in any outcome measure over the 10-week period. No significant (p˃0.05) strength or functional improvements were observed in the CG, though the EG demonstrated significant improvements in their 1RM leg press (p = 0.004) and chest press (p = 0.032), maximal push ups (p = 0.032), and global QOL (p = 0.011). The EG reported a significant increase in fatigue (p = 0.014), while the CG reported significant positive changes in anxiety measures (p = 0.005). Conclusion: The exercise intervention produced superior improvements in strength and global QOL, compared with the CG. Regardless of group allocation, enrolment in the exercise study appeared to mitigate the treatment-related decline expected in AYA undergoing cancer treatment.