August Cartwright had presumably killed him and taken his place around 2014.
Campbell worked as a plastic surgeon in Gotham City since 2000, becoming extremely beloved, well-known and respected among his fellow Gothamites for both his liposuctions and his philanthropic work. He was also one of the most renowned teachers at Gotham University.
At some point in his life Campbell married.
It is possible that he treated Duela Dent after she slashed her own face, without the patient's consent, as she was brought to the surgeon's clinic by her mother; since that happened in 2011 and it's unclear if it was the real Campbell as it should have been or August posing as him, as he claimed.
Around 2014, Campbell took a sabbatical year. This happened to be around the same time August Cartwright mysteriously disappeared. However, the latter actually resurfaced in Gotham City as Ethan Campbell, likely having killed him and taken his place.