Michael Hanson was head of the Security Service. When the Director General of MI5, Veronica Halliwell, was killed in a car accident, he was appointed to replace her.
However, Halliwell's death was not accidental. As part of their plan to take over Earth, the Voracians wanted to gain control of MI5. Knowing that Hanson was next in line for Halliwell's position, they converted him into a Voracian, then arranged for Halliwell's accident Hanson was killed when the Voracian mothership was destroyed due to the Fourth Doctor's version of Voractyll disrupting the mothership's systems. (PROSE: System Shock)