He was born in the Season 11 finale and the final episode of Shameless.
Frank was initially unaware that Monica was pregnant as he believed she had had an abortion some months previously. Monica who was pregnant with twins had the abortion but didn't do it on the NHS instead going to 'Cherokee Linda'. The termination was only partly successful with only one of the babies surviving - Ben. His twin William was successfully aborted.
Frank only discovered the truth when Monica suddenly went into labor in the Jockey and a horrified Frank did what he always did when faced with a shock... Sit down, have a fag and let someone else deal with it.
In the days after his birth, he was ignored by Frank and neglected by his depressed mother while tended to by his sister Stella. He met his brothers Lip and Carl as well as his sister Fiona who realizing the true situation at the Gallagher home, becomes determined to help her two youngest siblings escape the life they're doomed to endure in Frank and Monica's care.
Despite being left with Kev Ball by Fiona temporarily, Stella decides to walk out with her brother in her arms and find their father. They finds Frank in the street, after his spat with Monica, where he claims to blame Stella and Ben, as well as all his other kids, for ruining his life, and tells her that he's leaving. Saddened, Stella complies with his request to leave with Ben.
Afterwards, a guilty Frank finds Ben and Stella outside a park, where tries to apologize for his earlier words to them. Ben is held by Stella as she refuses to go since he told them to go away as women from the park think Frank is harassing the children. Stella says Frank isn't their father as he explains that she is in a mood but she says that he isn't since her father would love her and her brother. After Stella sees that he is honest in his apology when he states he does love them and goes on a swing while leaving him with Ben, while Frank explains the truth to the women who realize the situation and apologize to him. Holding his son, Frank expressed surprise at his newborn son before he changes his diaper.
They return home together with Fiona, Carl, and Lip entrusting Frank and Monica with their siblings after seeing they've changed.