The 17-year-old, whose chances have been limited since he became the club's youngest-ever player in 2008, stepped off the bench to fire his side to victory with a 60th-minute thunderbolt as the Lions ended their campaign with a whimper.
Millwall almost snatched a point at the death when Tamika Mkandawire saw his header come back off the upright, but it was the Tykes who held on to claim the win that secures their best finish since returning to the Championship five years ago.
Barnsley striker Marlon Harewood failed to recover from a hamstring injury in time to play, while Danny Rose was only named on the bench following his knee strain.
Neil Harris returned from a ban but had to settle for a place on the bench in a Lions side showing five changes from the 2-0 defeat against Swansea that ended their play-off hopes, with Josh McQuoid partnering Steve Morison in attack.
There was a real end-of-season feel to the game early on, with both sides looking short of ideas in the final third.
Morison spurned the best chance of a tame opening after 24 minutes when he forced Luke Steele into a decent save low down to his left.
Andros Townsend followed suit soon after with a shot from distance, with the Barnsley defence guilty of backing off.
The Tykes had to wait until five minutes before the break to test Millwall goalkeeper David Forde for the first time, but he was up to the task as he parried away Jason Shackell's glanced header before on-loan Townsend had another stinging shot saved at the other end.
Needing to inject some life into his toiling attack, Barnsley boss Mark Robins opted to introduce Noble-Lazarus for Jacob Mellis at the break - and within 15 minutes the youngster had made a stunning impact.
Jacob Butterfield had already forced Forde into a good parried save with a long-range effort before youngster Noble-Lazarus took centre stage by collecting Nathan Doyle's pass and cutting inside before drilling left footed from outside the area with a superb shot that beat Forde.
The Lions responded immediately with a double substitution, as boss Kenny Jackett opted to replace Hameur Bouazza with James Henry and McQuoid with John Marquis.
It almost had the desired effect too, with Marquis forcing Steele to hold onto a snapshot from close range with virtually his first touch of the ball before Morison blasted wide.
It was the Tykes who were now in the ascendancy, though, with the pace of Noble-Lazarus in particular causing all sorts of problems for Millwall's defence.
Doyle came close to a second goal with 13 minutes left when he lashed the ball wide after working space well, before the Lions almost snatched a late leveller.
In the second of six minutes of injury time, Mkandawire was unlucky to see his header from James Henry's cross cannon back off the post, but it mattered little as Barnsley held on to secure 17th place in the npower Championship.