Background Persons seeking emergency injury care are often from higher-risk and underserved key populations (KPs) and priority populations (PPs) for HIV programming. While facility-based HIV Testing Services (HTS) in Kenya are effective, emergency department (ED) delivery is limited, despite the potential to reach underserved persons. Methods This quasi-experimental prospective study evaluated implementation of the HIV Enhanced Access Testing in Emergency Departments (HEATED) at Kenyatta National Hospital ED in Nairobi, Kenya. The HEATED program was designed as a multi-component intervention employing setting appropriate strategies for HIV care sensitization and integration, task shifting, resource reorganization, linkage advocacy, skills development and education to promote ED-HTS with a focus on higher-risk persons. KPs included sex workers, gay men, men who have sex with men, transgender persons and persons who inject drugs. PPs included young persons (18–24 years), victims of interpersonal violence, persons with hazardous alcohol use and persons never HIV tested. Data were obtained from systems-level records, enrolled injured patient participants and healthcare providers. Systems and patient-level data were collected during a pre-implementation period (6 March − 16 April 2023) and post-implementation (period 1, 1 May − 26 June 2023). Additional, systems-level data were collected during a second post-implementation (period 2, 27 June − 20 August 2023). HTS data were evaluated as facility-based HIV testing (completed in the ED) and distribution of HIV self-tests independently, and aggregated as ED-HTS. Evaluation analyses were completed across reach, effectiveness, adoption, implementation and maintenance framework domains. Results All 151 clinical staff were reached through trainings and sensitizations on the HEATED program. Systems-level ED-HTS among all presenting patients increased from 16.7% pre-implementation to 23.0% post-implementation periods 1 and 2 (RR = 1.31, 95% CI: 1.21–1.43; p < 0.001). Among 605 enrolled patient participants, facilities-based HTS increased from 5.7% pre-implementation to 62.3% post-implementation period 1 (RR = 11.2, 95%CI: 6.9–18.1; p < 0.001). There were 440 (72.7%) patient participants identified as KPs (5.6%) and/or PPs (65.3%). For enrolled KPs/PPs, facilities-based HTS increased from 4.6% pre-implementation to 72.3% post-implementation period 1 (RR = 13.8, 95%CI: 5.5–28.7, p < 0.001). Systems and participant level data demonstrated successful adoption and implementation of the HEATED program. Through 16 wk post-implementation a significant increase in ED-HTS delivery was maintained as compared to pre-implementation. Conclusions The HEATED program increased overall ED-HTS and augmented delivery to KPs/PPs, suggesting that broader implementation could improve HIV services for underserved persons already in contact with health systems.