Background Acute postoperative pain after breast cancer surgery adversely affects recovery and is an independent predictor of chronic postsurgical pain in these patients. Serratus plane blocks have been found to provide analgesia to the anterior hemithorax. However, trials comparing superficial serratus plane block and deep serratus block in breast cancer surgery patients are sparse. Methodology A total of 74 female patients with American Society of Anesthesiologists physical status I and II scheduled for elective modified radical mastectomy for breast cancer were randomized into two groups. Group A patients received a superficial serratus plane block with 30 mL of 0.25% bupivacaine, and group B patients received a deep serratus plane block with 30 mL of 0.25% bupivacaine. Postoperatively, the Numerical Rating Scale (NRS) score was measured during the immediate postoperative period, after 30 minutes and at one, four, eight, 16, and 24 hours, as well as on the second and third day. After discharge, the NRS scores were recorded in the second and third weeks and then monthly once for three months. All patients received patient-controlled analgesia with intravenous (IV) morphine. The duration of analgesia, pain scores, and 24-hour morphine consumption were also noted. Results In group A, the mean duration of analgesia (hours) was 5.51 ± 1.42, whereas in group B the mean duration of analgesia (hours) was 6.69 ± 1.18 (p < 0.01). NRS scores for pain during rest at 12 and 16 hours and NRS scores for pain during cough at eight, 12, and 16 hours, as well as at the third month were significantly lower in group B. However, morphine consumption was comparable between the groups. Conclusions Deep serratus plane block was associated with better NRS scores for pain on rest and coughing and prolonged duration of analgesia after a modified radical mastectomy. We conclude that the deep serratus plane block provides superior and extended analgesia than the superficial serratus plane block after a modified radical mastectomy.