The mosaic leafhopper, Orientus ishidae (Matsumura), is an Asian species widespread in Europe that can cause leaf damage in wild trees and transmit disease phytoplasmas to grapevines. Following an O. ishidae outbreak reported in 2019 in an apple orchard in northern Italy, the biology and damage caused by this species to apples were investigated during 2020 and 2021. Our studies included observations on the O. ishidae life cycle, leaf symptoms associated to its trophic activity, and its capability to acquire “Candidatus Phytoplasma mali,” a causal agent of Apple Proliferation (AP). The results indicate that O. ishidae can complete the life cycle on apple trees. Nymphs emerged between May and June, and adults were present from early July to late October, with the peak of flight between July and early August. Semi-field observations allowed for an accurate description of leaf symptoms that appeared as a distinct yellowing after a one-day exposure. In field experiments, 23% of the leaves were found damaged. In addition, 16–18% of the collected leafhoppers were found carrying AP phytoplasma. We conclude that O. ishidae has the potential to be a new apple tree pest. However, further studies are required to better understand the economic impact of the infestations.