The former Sheffield and Matlock Town boss joined the club back in November 2017.
Manager Steve Kittrick said: "I have decided I want to take the club in a different direction, but would like to thank Mark for his time and effort while at the club.”
The Seadogs had been setting the pace in the EVO-STIK League Premier Division, but have now lost six of their last eight league games, including a 2-0 reverse at local rivals Whitby Town on New Year’s Day.
Keep up-to-date with our exclusive email newsletters.
Subscribe