Gudjon Thordarson's side needed to win and rely on other results going their way to avoid dropping into the basement.
But Leicester refused to oblige and rammed three second-half goals past a bedraggled outfit.
Strike ace Matt Fryatt sealed an excellent campaign for both himself and his club, who are making an immediate return to the Championship, when he grabbed his 32nd goal.
Fryatt drew home keeper Steve Collis and rolled the ball home two minutes after Lloyd Dyer's toe-poked effort went in off the post from the edge of the box.
Before that Bruno Berner's lazy long-range strike had bounced past Collis at his near post to kick-start Leicester's easy victory.
But they may have fallen behind early in the first half when Icelandic midfielder Gylfi Sigurdsson had a shot cleared off the line by Matty Oakley.
And seven minutes after the interval Alex midfielder James Bailey clattered the crossbar with a deflected shot and Anthony Elding's volleyed finish was chalked off for offside.
But Leicester were largely in control of the game and Steven Howard might have made the margin of victory greater had he taken a couple of first-half chances.
The big striker skied over after Andy King had set him up in front of goal and then, after climbing the highest at the far post, he thumped his header into the side-netting.
Leicester's assistant manager, Craig Shakespeare, said after the win: "League One has been new to us and we've come to grounds we've not experienced before, but we're really looking forward to going back to playing in the Championship next season."
Crewe boss Thordarson pledged to stay on at Gresty Road and promised a promotion campaign for the Railwaymen next year.
"Leicester are the best team in this League and they were too good for us today," he admitted.