The Foxes put the seal on 10-man Boro's disappointing Championship season with a deserved victory at the Walkers Stadium.
Yann Kermorgant's wonder goal shortly before half-time set the Foxes on their way, before Richie Wellens rounded things off with a penalty on his 400th career appearance.
The result brought about Boro's first-ever loss at the Walkers and just their second in 11 games, but they could have taken at least a point had Chris Killen converted a second-half penalty of his own, only for him to miss and then watch on as keeper Brad Jones was later sent off.
Anything other than a win would have been tough on Leicester, though, who were highly impressive throughout an entertaining contest.
They now head into a two-legged duel with Cardiff, while Gordon Strachan and his coaching staff face a tough summer after failing to deliver the instant return to the Premier League that was expected of them.
Leicester were handed a pre-match boost when leading scorer Matt Fryatt was named amongst the substitutes, signalling the end of his three-month stay on the sidelines with a broken jaw.
Fryatt's side were the first to settle and enjoyed the better of the opening half.
Wellens was impressive in midfield, while James Vaughan tried to open the scoring with a scuffed scissor kick.
The on-loan Everton man continued to threaten and was soon through on goal, only for Jones to smother the ball at his feet, before jumping up to save from Paul Gallagher on the rebound.
The chances continued to come for Leicester and Kermorgant wasted one of them, nodding wide from a Gallagher cross, before the Frenchman hit the back of the net four minutes before the break.
It was his first goal for the club and he left Jones with no chance as he curled a stunning effort in from 30 yards.
The pattern did not alter after the break, with Leicester pouring forward in numbers, and Jones was forced into action again as he tipped Dany N'Guessan's drive over.
Fryatt came on just shy of the hour, before Middlesbrough registered their first meaningful effort after 67 minutes, with Gary O'Neil's strike comfortably dealt with by Conrad Logan.
Boro then had a glorious chance to level, albeit against the run of play, when Wellens handled in the box, only for Killen's penalty to be beaten away by Logan.
The reserve team keeper was involved again shortly after, this time saving well from Kyle Naughton, before at the other end, Jones saw red after bringing down Lloyd Dyer with six minutes remaining.
Wellens duly tucked away the resulting spot-kick to put the seal on the win.