Mark didn't do anything to merit his title, he didn't even have to trouble himself to do anything apart from survive his father. Margaret Thatcher's husband, Denis was given a peerage in 199- and became Baron Thatcher of Scotney. Unlike almost all created peers these days, he was given a life peerage but a hereditary one (the only non-member of the royal family to get one in ages). Mark simply inherited the peerage when his father died - hence 'Sir' Mark Thatcher.
Mrs Thatcher was also given a life peerage in her own right in 1992.
I leave you to guess the kind of titles attached to members of the Thatcher family in South Yorkshire, but it's not 'sir' or 'lady'.