The 58-year-old was last seen at around 6am this morning (Saturday 12 June) on Portswood Road.
Police are growing increasingly concerned for his safety and are now asking the general public to report any sightings to us.
Mark is described as white, approximately 5foot 9inches tall, of skinny build, bald and with no teeth.
Mark was last seen wearing black framed glasses and a red baseball cap. He also had a blue wheeled walker with him.
If you have seen Mark today, please call police on 101 quoting incident 44210229881.
Dial 999 in the event of an emergency