He eventually graduated from college, along with Daniel, and lived near his hometown so he could visit and play basketball with his brothers. One time, while visiting with Daniel, he got his two younger brothers to spy with him on Daniel, who was preparing to make love with his girlfriend, Alice Jardine. Just as he was stripping her bra off, Sean yelled, "Danny, don't do it, you're a young man!" This startled Alice, who had her shirt over her eyes, and knocked herself out by running into a wall. Angry, Daniel grabbed a shovel, and tried hitting Sean with it. Sean yelled, "What're you trying to hit me for? I just did you a favor", which made Daniel more angry, causing him to throw the shovel at Sean, but miss and hit a kerosene lantern, which nearly set the barn on fire.
Daniel would later head off to basic training the next day, and later Sean and his younger brothers would join. They were all assigned to Able Company, 1st Battalion, 116th Infantry Regiment, 29th Infantry Division. However, after all of the Sullivan brothers died on the Juneau, a law was passed in the army that no two brothers should stay in the same unit, so Sean's brothers were all assigned to different units.
He would participate in the Invasion of Normandy, set on June 6, 1944. He landed in the first wave on Omaha Beach. He was eventually killed while trying to get up the beach. His corpse was seen after the battle.
Sean Ryan's final resting place is presumably in the Normandy American Cemetery and Memorial alongside the thousands of the fallen, among them most notably, Captain John H. Miller.