Background: Clinical practice suggests that objective assessment tools are needed to assess adults with inattention or hyperactivity, informed by the underlying pathophysiology of attention-deficit and hyperactivity disorder (ADHD). This systematic review comprehensively evaluates the current objective assessment methods as an adjunct diagnostic tool for these adults. Methods: We conducted a systematic review of studies investigating various objective diagnostic methods to assess adults with ADHD and healthy controls. The database search occurred from its inception to 23 December 2024. Results: Our search yielded 46 studies that reported on various objective methods to assess adults with ADHD. The MOXO-distracted Continuous Performance Test (MOXO-d-CPT), eye-tracker with MOXO-d CPT, Conners’ Continuous Performance Test—3rd edition (CCPT-3), and oculomotricity can differentiate between true and feigned ADHD or other diagnostic possibilities. The Quantified Behavior Test (Qb Test+) can detect hyperactivity and differentiate it from other psychiatric disorders. Mono-d, CCPT-3, Qb Test+, Test of Variables and Attention (TOVA), Integrated Visual and Auditory Continuous Performance Test (IVA-CPT), and oculomotricity can monitor pharmacotherapy response. Functional near-infrared spectroscopy (fNIRS) offers more promise than structural imaging and demonstrates a moderate level of sensitivity and specificity to differentiate adults with and without ADHD by performing the verbal fluency test. Notwithstanding, electroencephalography (EEG)/event-related potential (ERP) shows potential in diagnosis and treatment monitoring (e.g., neurofeedback training). In addition, transcriptome-based biomarkers have also been explored as diagnostic tools. Conclusion: The diagnosis and monitoring of ADHD in adults come with a unique set of challenges due to psychiatric comorbidity, including depression and anxiety; fluctuation of symptoms over time; and lack of consensus among clinicians and professional organizations to adopt objective tests in the diagnostic process. Our findings support the notion that a combination of clinical assessment and objective biomarkers targeting distinct pathophysiological aspects may enhance the accuracy of ADHD diagnosis.