A concentrated multijunction solar cell offers good promise for high energy density. In a concentrated photovoltaic system, the solar cell's output power depends not only on the illumination energy but also on the spectral distribution and the uniformity of illumination. Under field operating conditions, non-uniformity is a major factor affecting the performance of concentrated solar cells. The present work is intended to address non-uniform illumination created on concentrated commercial triple junction solar cells. The study considered non-uniformity (both spatial and spectral) based on two cases (varying focal length and masking cell area). Both methods create non-uniformity, which affects the performance of triple-junction solar cells and is addressed here with experimental evaluation. The shadowing factor is a critical parameter considered for performance evaluation theoretically and experimentally. A Fresnel-based concentrating (commercial) photovoltaic system with InGaP/InGaAs/Ge triple-junction solar cells is employed for the purpose. The experimental results indicate that there is an 8 % ± 3 % reduction in overall efficiency with varying non-uniformity under 500-700 W/m2. The main parameters that are affected significantly are maximum power point voltage and current. Also, the influence of series resistance under increased non-uniformity plays a vital role in performance reduction. The experimental results are fitted to a proposed simple lumped diode model.