Vardy's first-half penalty and Maguire's header after the break secured a 2-0 victory for Leicester at St James' Park, their third in succession on Tyneside, with Ashley among a crowd of 51,523 for the first time since May 2017.
The sportswear magnate witnessed in graphic detail the problems his transfer policy has created with the Magpies' decision not to invest in a proven goalscorer coming back to haunt them as chances went begging.
Kenedy, arguably their most creative player, had to line up at left-back with Paul Dummett injured and no other specialist in the squad.
Ashley, who had arrived at the stadium hours after a report claimed former Manchester United and Chelsea chief executive Peter Kenyon is attempting to raise finances to launch a takeover bid, headed home with television cameras having captured him smiling as sections of the home fans sang "Get out of our club" and "Stand up if you hate Ashley".
Both sides had early chances in a lively start to the game with Wilfred Ndidi doing well to block Ayoze Perez's shot from a DeAndre Yedlin pull-back and Kelechi Iheanacho firing straight at Martin Dubravka after being allowed to run in on goal.
Ndidi headed over from an eighth-minute James Maddison corner with the visitors playing the more fluent football, and Dubravka had to get down at his near post to turn aside Ricardo Pereira's well-struck shot two minutes later.
The Foxes were dominating the midfield battle with Ndidi and Nampalys Mendy allowing Maddison and Ricardo to probe, although the Magpies passed up a glorious 18th-minute opportunity when Perez played in Joselu only for the striker to delay his shot and allow Maguire to intervene.
Mohamed Diame headed wide when he might have done better from Christian Atsu's inviting cross four minutes later, but Maguire did hit the target at the other end from Maddison's 28th-minute corner, prompting Dubravka to make another save.
However, the Slovakia international was beaten two minutes later when, after referee Simon Hooper had ruled that Yedlin had blocked Maguire's goal-bound effort with his arms, Vardy was clinical from 12 yards.
Jonjo Shelvey tested the back-pedalling Kasper Schmeichel from halfway with an audacious effort eight minutes before the break.
But Newcastle boss Rafael Benitez turned away in disgust four minutes after the restart when Hooper waved play on after Perez appeared to have been wrestled to the ground by Maguire inside the box.
The perceived injustice seemed to inspire the home side as the crowd responded, although Maguire dragged a 59th-minute shot across goal at the other end after miskicking his initial volley.
But their hopes of mounting a fightback were dashed when Maguire climbed to power home Maddison's 73rd-minute corner and cement the victory.