State-of-the-art X-ray breast imaging (BI) modalities such as digital breast tomosynthesis (DBT), contrast enhanced spectral mammography (CESM) and breast CT (BCT) impose demanding requirements on digital X-ray detectors. This work studies the imaging performance of a GaAs two-threshold photon-counting detector (PCD) prototype for BI relevant X-ray spectra. The prototype has a 75 μm pixel size, two calibrated energy thresholds from 8 to 60 keV, 8 x 4 cm2 area and a 0.5 mm thick GaAs sensor. The X-ray spectra used were 28 and 35 kVp with 2 mm Al filtration from a W-target tube emulating RQA-M2. The main imaging metrics probed include modulation transfer function (MTF) and detective quantum efficiency (DQE). Air kerma spanned three orders of magnitude from 370 nGy to 330 μGy. Furthermore, the detector’s linearity, lag and ghosting were also tested. For 28 kVp, the GaAs PCD exhibits 85% and 48% DQE for 0 and 5 lp/mm respectively, independent of the applied dose. MTF ranges from 98% to 53% for 1 and 6.667 lp/mm (Nyquist limit). Excellent linearity, zero lag and ghosting were observed. GaAs PCD technology is an ideal candidate for BI detector panels. Its stable temporal behavior, inherent zero readout noise and excellent DQE independent of the applied X-ray dose, improve on the combined advantages of current CsI/CMOS and a-Se/a-Si BI detectors. In addition, the multiple energy thresholds can enable spectral single-shot methods without motion blur.