Globally, unmet need for postpartum family planning is high. However, immunization services are among the most widely utilized health services. Establishing systematic screening, counseling, and referral systems from different contact points, particularly from EPI units may improve postpartum family planning uptake. Hence, this study aimed to assess the effect of counseling for family planning at EPI units on contraceptive uptake during the extended post-partum period. A before-and-after type of quasi-experimental study was conducted in 8 purposively selected primary health care units in Sidama region, Ethiopia. All mothers visiting the selected health facilities for infant immunization services from February 06 to August 30, 2020, were screened, counseled, and referred for family planning. A structured interviewer-administered questionnaire was used to collect data from 1421 randomly selected mothers (717 for pre-intervention and 704 post-intervention phases). EpiData version 3.1 and SPSS version 22 were used for data entry and analysis. The effect of the intervention was assessed using multivariable logistic regression analysis adjusting for the effects of potential confounders. P value < 0.05 was considered statistically significant. The contraceptive utilization rate before intervention was 72.7% with 95% CI (69.5, 75.9). It was 91.9%, 95% CI (89.8%, 93.9%) after the intervention. Utilization of contraceptive pills increased from 4.3% to 6.9%, injectables from 52.4% to 57.5%, implants from 12.8% to 22.9%, and IUCD from 3.2% to 5.0% after the intervention. After adjusting for the effect of possible confounding variables, screening, counseling, and referring mothers for family planning at infant immunization units significantly increases the contraceptive utilization rate among mothers presented for infant immunization services(AOR = 5.83, 95% CI: 4.02, 8.46). Screening, counseling, and referring mothers for family planning services at infant immunization units significantly increases postpartum contraceptive uptake. Integrating family planning messages with infant immunization services is recommended. Trial registration: ClinicalTrials.gov Identifier: NCT04767139 (Registered on 23/02/2021).