Preston's first chance came after four minutes when David Jones had a left-foot shot from the right channel saved from 20 yards by Hull keeper Boaz Myhill.
Chris Sedgwick hit a 25-yard shot after 17 minutes which was tipped over the bar for a right-wing corner.
Callum Davidson fired over a left-foot cross and Jones had a header tipped around the left-hand post for a corner, before Johnson hit the left-hand post with a shot and Sedgwick hit the rebound goalbound, but the ball was cleared off the line.
Johnson scored the opener after an hour, when home skipper Chris Lucketti played a through ball to the on-loan striker and he beat the offside trap and pulled the ball inside from the by-line and fired into the roof of the net from six yards out.
Peter Taylor made a double change, with Curtis Woodhouse and Mark Lynch being replaced by Stuart Green and Nicky Barmby.
Youl Mawene replaced Claude Davis after 65 minutes and in the 69th minute there was a loud cheer for the return of David Nugent, who came on as a replacement for Patrick Agyemang.
In the 73rd minute Jones, on a season-long loan from Manchester United, fired a loose ball in off the post from 20 yards to make it 2-0 to Preston.
Paul McKenna wrapped things up for Preston when he ran through the middle of the pitch and then unleashed a 25-yarder into the top right-hand corner of the net to make it 3-0.