Management of invasive aspergillosis has been improved by biomarker assays, but limited accessibility and batch testing limit the impact. Lateral flow assays (LFA) are a simple method for use outside specialist centers, provided performance is acceptable. The objective of this study was to determine the performance of the recently released IMMY sona Aspergillus LFA when testing serum samples. The study took the form of a retrospective, anonymous case/control study comprising 179 serum samples from 136 patients with invasive fungal disease, previously documented using recently revised internationally accepted definitions. The LFA was performed following the manufacturer's instructions using a cube reader to generate a galactomannan index (GMI). Performance parameters were determined, and receiver operator characteristic (ROC) analysis was used to identify an optimal threshold. Concordance with the Bio-Rad Aspergillus Ag assay (GM-EIA) was performed. At the recommended positivity threshold (GMI ≥ 0.5), LFA sensitivity and specificity were 96.9% (31/32) and 98% (98/100), respectively. ROC analysis confirmed the optimal threshold and generated an area under the curve of 0.9919. Qualitative agreement between LFA and GM-EIA was 89.0%, generating a Kappa statistic of 0.698, representing good agreement, with most discordance arising due to false-negative GM-EIA samples that were positive by LFA. The median GMI generated by the LFA was significantly greater than that generated by the GM-EIA. The IMMY sona Aspergillus LFA, when used with a cube reader, provides a rapid alternative to the well-established GM-EIA, potentially detecting more GM epitopes and enhancing sensitivity.