Purpose Female adolescents seeking emergency department (ED) care are at high risk of unintended pregnancy, primarily because of contraceptive nonuse; yet, few ED patients follow up for reproductive care when referred. The objective of this cohort study was to determine the feasibility, acceptability, adoption, fidelity, and potential efficacy of a personalized and interactive ED-based pregnancy prevention mobile health intervention (Emergency Room Interventions to improve the Care of Adolescents [Dr. Erica]). Methods We conducted a prospective cohort study with sexually active female ED patients aged 14–19 years who were not using highly effective contraceptives. Dr. Erica consists of a 10-week, automated, two-way texting intervention based on an evidence-based sexual health curriculum, the Social Cognitive Theory, and motivational interviewing techniques. At 12 weeks, we conducted follow-up via online survey and phone call to measure feasibility, acceptability, adoption, fidelity, and preliminary efficacy data (contraception initiation). Results We screened 209 female ED patients to enroll 42. The average age was 17.5 years (standard deviation ± 1.4); the majority were Hispanic (n = 37, 88%) and had a primary provider (n = 40, 95%). One participant opted out (1/42, 2%), and a total of 35 participants (83%) completed follow-up. Although interactivity diminished with time, 83% of participants (35/42) replied to one or more text. Ninety-four percent of participants (29/31) liked the messages, and 83% (25/30) would recommend the program. Hormonal contraceptives were initiated by 46% of participants (16/35). Conclusions Dr. Erica was feasible and acceptable among female adolescent ED patients and demonstrated high fidelity and adoption. The intervention also showed potential to increase highly effective contraceptive use among high-risk females.