Atrazine is often used during the establishment of centipedegrass for control of annual weeds such as crabgrass (Digitaria spp.). Tank-mixing of mesotrione with photosynthesis inhibiting herbicides such as atrazine have been shown to be synergistic. Research was conducted to evaluate the effect of atrazine and mesotrione, both alone and in tank-mixture, on centipedegrass photosystem II (PSII) efficiency when applied 2 week after emergence from seed. Atrazine was applied at 0.28, 0.56, 1.1, and 2.2 kg/ha. Mesotrione was applied at 0.035, 0.07, 0.14, and 0.28 kg/ha. Tank-mixtures included atrazine at 2.2 kg/ha plus the four mesotrione rates and mesotrione at 0.28 kg/ha rate plus the four atrazine rates. PSII efficiency was measured as quantum yield of PSII with a pulse-modulated fluorometer under light-adapted conditions. Mesotrione alone at the highest rate of 0.28 kg/ha reduced PSII efficiency at 6 days after treatment (DAT), but no visual injury symptoms were observed and PSII efficiency normalized by 10 DAT. No other mesotrione alone treatments reduced PSII efficiency. All atrazine treatments (0.28 to 2.1 kg/ha), whether alone or in tank-mixture, reduced PSII efficiency of centipedegrass for 28 days. Tank-mixtures of mesotrione plus atrazine were more deleterious on centipedegrass development compared to alone applications. Atrazine at 2.2 kg/ha plus mesotrione 0.28 kg/ha reduced PSII efficiency up to 90% which translated into up to 70% growth reduction. However, while PSII efficiency and growth were reduced, few phytotoxic effects were observed for any treatment.