Background: Postpartum application of an intrauterine device (IUD) is often challenging to many women and induces fear, especially when performed without general anesthesia. This renders insertion difficult, which may predispose to complications, or the women to delay her decision for contraception. Our study compared the complications of Mirena IUD insertions during cesarean section (CS) versus 42 days postpartum. Methods: This study is a prospective cohort comparative study conducted in a private hospital in Saudi Arabia, from May 2021 to December 2021. Women were assigned into two groups. The first group (post-placental) contained 48 women in whom Mirena IUDs were inserted during CS, after placental delivery. The second group (postpartum) included 48 women where the intrauterine contraceptive device (IUCD) was inserted after 42 days postpartum. The primary outcome was the expulsion rate of IUD, while secondary outcomes were infection, perforation, bleeding, and displacement. The patients were followed up one month and three months after Mirena’s insertion. Results: There was no significant difference between the two groups regarding the expulsion rate (p = 0.646). Also, there was no significant difference between the two groups in terms of secondary outcomes (p > 0.05). Conclusions: Post-placental application of Mirena IUD is more accessible, more convenient, and less fearful for the patients.