Introduction: There is a lack of information on illicitly manufactured fentanyl and fentanyl analogue-related (IMF) unintentional overdose death trends over time. The study analyzes IMF-related unintentional overdose fatalities that occurred between July 2015 and June 2017 in Montgomery County, Ohio, an area with the highest rates of unintentional overdose mortality in Ohio. Methods: LC–MS/MS-based method was used to identify fentanyl analogs and metabolites in 724 unintentional overdose death cases. The Chi-square statistic was used to assess differences over time in demographic and drug-related characteristics. Results: The number of unintentional overdose death cases testing positive for IMFs increased by 377% between second half of 2015 and first half of 2017. The majority of decedents were white (82.5%) and male (67.8%). The proportion of fentanyl-only (no other analogs) cases declined from 89.2%–24.6% (p < 0.001), while proportion of fentanyl analogue-containing cases increased from 9.8%–70.3% (p < 0.001) between the second half of 2015 and first half of 2017. The most commonly identified fentanyl analogs were carfentanil (29.7%), furanyl fentanyl (14.1%) and acryl fentanyl (10.2%). Proportion of IMF cases also testing positive for heroin declined from 21.6% to 5.4% (p < 0.001), while methamphetamine positive cases increased from 1.4%–17.8% (p < 0.001) over the same time period. Discussion: Emergence of fentanyl analogs contributed to substantial increases in unintentional overdose deaths. The data indicate a growing overlap between the IMF and methamphetamine outbreaks. Continuous monitoring of local IMF trends and rapid information dissemination to active users are needed to reduce the risks associated with IMF use.