Hibs took the lead on Saturday through Richie Towell at McDiarmid Park but goals from Francisco Sandaza, Marcus Haber and Dave Mackay sealed the win for the Perth Saints.
Caretaker boss Billy Brown was in charge of team affairs for this game but Fenlon - appointed to the Easter Road hotseat on Friday - would have been far from impressed from his vantage point in the directors' box.
Hibs were without Garry O'Connor after the striker failed to recover from illness in time.
Cillian Sheridan recovered from a hamstring strain to start in what was Saints boss Lomas' first home match at the helm but Jody Morris was out with injury and a virus.
However, the home side were dealt an early blow when Sheridan pulled up and appeared to suffer a recurrence of the injury while chasing a Sandaza pass.
He was forced out of the action with just 10 minutes on the clock and replaced by Haber.
Real chances in front of goal were few and far between early on but Hibs surged into the lead after 26 minutes despite the best efforts of the Saints goalkeeper.
Peter Enckelman blocked well from a point-blank shot from Leigh Griffiths and got a hand to Towell's follow-up shot but could not prevent the rebound from hitting the back of the net.
Martin Scott then missed a headed chance for the visitors before Steven Anderson blasted high and wide at the other end, with Graham Stack untroubled in the Hibs goal.
Chris Millar saw his effort flash across the face of goal and wide as the home side continued to press for the equaliser.
The hard work paid off and Saints were back on level terms after 39 minutes when Sandaza rammed home the rebound after his first attempt crashed off the post.
Saints took the lead on the stroke of half-time when Mackay picked out Haber at the back post and he scored with a free header from close range.
St Johnstone increased their advantage two minutes after the restart when Mackay stepped up for a free-kick and sent a curling effort past Stack and into the back of the net in off the post.
Griffiths then squandered the chance to pull a goal back for Hibs from a David Wotherspoon cross but the forward was offside anyway.
Saints still looked capable of finding the net again and Sandaza had a go from the edge of the box but he was off-target on this occasion and rifled over.
St Johnstone were still hungry for goals and were threatening again when Sandaza turned provider for Murray Davidson but his effort was tame and comfortably gathered by the Hibs goalkeeper.
Hibs passed up a decent chance themselves when Danny Galbraith whipped a cross into the path of Junior Agogo and his header dipped inches over the crossbar.
But it was too little too late from the Edinburgh side as they returned to the capital empty handed, with just three wins so far in the SPL this season.