Mitchell was security chief at the Kyme Institute.
He arrived at the landing site of Missy's TARDIS when she arrived at the Institute with Yayani and ordered the two to stand down. Yayani gave up her staser, but when Mitchell moved forward to arrest the two, Missy activated the blast doors, which severed Mitchell's arm and afforded the two access to the terminal. Mitchell quickly died from his wound. (PROSE: Lords and Masters)