Parkin rose to head home 14 minutes from the end of a passionate Yorkshire derby.
Hull frustrated the visitors for long spells and Parkin, who had earlier seen a goal controversially ruled out, was on the mark to give the hosts a deserved win.
Leeds, who have now failed to win any of their last five games, appear to have lost form at the worst time.
United started the game strongly though and a curling left-footed free-kick from Eddie Lewis forced Hull goalkeeper Boaz Myhill into a full-length save. At the other end, Parkin latched on to a flick-on to volley superbly into the top corner, but he was denied by the offside flag.
Jonathon Douglas blazed over an excellent chance from eight yards for Leeds and visiting goalkeeper Neil Sullivan had to be alert to parry Stuart Green's ten-yard drive.
Leeds twice went close to breaking the deadlock at the start of the second half, Shaun Derry firing just wide from 25 yards, before Liam Miller fluffed a clear opening moments later.
However, Hull always looked dangerous and, with time ticking away, Green lobbed in a good cross from the right and Parkin squeezed a far post header beyond Sullivan.
Leeds poured forward, but the closest they came to grabbing an equaliser was when substitute David Healy's drilled free-kick flew just wide in the final minute.