Matthew Sean Montgomery was conceived in the hopes that he would be a viable bone marrow donor for his older sister Molly. For a brief time it was believed that his father could be Sean Cudahy, which is where his middle name (Sean) came from. He lived with his parents and older sister in Seattle until his father's death, at which time it was just him, his mother, and his sister.
Erica was unpleasantly surprised to come home one day and find out that a young man had turned her house into party central. She ended the party, and he introduced himself as her nephew Sean (he was now going by his middle name). His sister Molly was in Tokyo doing an internship and Barbara had recently gone off to Europe with a new boyfriend. Erica wasn't pleased to have him there and wanted Jack to get him to leave. Sean was more than willing to go, telling Jack he had his own car and all his own credit cards, but Jack realized that Sean might be a good one to keep an eye on Lily. Jonathan also clued in and asked him to watch Lily for him. Sean soon got himself tangled up with Colby Chandler and her new nemesis Sydney, and was the guest of honor at the two's dual Sweet 16 party. At the party, he helped Lily escape to meet a boy she had met online, then he, Colby, and a few other guests took the party on board the Montgomery yacht. Although the harbor master eventually showed up and told them to put a stop to the party, it was after Sean and Colby had sex in the wheelhouse.