The emerald ash borer (EAB), Agrilus planipennis Fairmaire, a buprestid beetle native to Asia, has become a serious pest of ash trees (Fraxinus spp.) in North America since the early 2000s. Due to the impracticality of applying insecticides in natural forests, biocontrol is the most viable method to manage EAB in natural ecosystems. Here, we report the first evidence for the establishment and impact of Spathius galinae Belokobylskij & Strazenac, a larval parasitoid first released in North America in 2016 and 2017 at six mixed-hardwood forest sites, in Connecticut, New York, and Massachusetts. We also report current levels of abundance and parasitism of another introduced larval EAB parasitoid, Tetrastichus planipennisi Yang (Hymenoptera: Eulophidae), released in 2015 and 2016 in these same sites. Spathius galinae was recovered at all release sites in 2018, and its density in sampled trees had increased 1.5- to 20-fold (relative to the first postrelease sample year), reaching a final density of 2.3–14.3 broods/m² of phloem area and causing 13.1–49.2% marginal rate of parasitism at four of the six sites. In contrast, T. planipennisi was only recovered in 2018 at four of the six release sites, and both its density (0.1–2.3 broods/m² of phloem area) and parasitism (0.1–5.6%) were lower than that of S. galinae throughout the study at the four sites where recoveries were made. Our data fill a critical gap in the development of a biocontrol-based EAB management plan to protect surviving ash trees capable of reaching maturity and producing replacement trees.