McLean finished from the edge of the box midway through the second half and St Mirren survived Marc McAusland's 77th-minute red card without any major scares.
Hibs, without top goalscorer Garry O'Connor, suffered another blow to their SPL survival hopes in injury-time when Lewis Stevenson was sent off.
Hibs remain just three points ahead of bottom club Dunfermline, who secured their first home win of the season 24 hours earlier against Aberdeen, and key midfielder Stevenson will now miss Wednesday's trip to Pittodrie.
Fenlon's men have lost both games without scoring since their Scottish Cup semi-final win over the Dons. O'Connor was missing through illness and his replacement, Eoin Doyle, failed to convert their two best chances.
Fenlon was also frustrated over referee Stephen Finnie's failure to award a penalty for Dougie Imrie's challenge on Leigh Griffiths, but his side were short of creativity and width in particular with Ivan Sproule demoted to the bench until the latter stages.
The match started with both sides getting forward and Griffiths forced a near-post save from Craig Samson before team-mate George Francomb volleyed well over from 18 yards after the ball fell to him following a corner.
Saints were getting forward quickly through Paul McGowan and Nigel Hasselbaink and they created the best chance of the opening quarter when Steven Thompson headed Graham Carey's deep inswinging corner just wide.
The game tightened up but Griffiths almost lifted the scrappy proceedings with a powerful 30-yard strike, but failed to hit the target.
St Mirren had a decent spell of pressure but they were almost caught out with a simple ball down the channel from Hibs right-back Matt Doherty.
Doyle ran on to it unchallenged and lifted the ball over the onrushing Samson, but he could not direct it on target.
The Irishman had a better chance after the break. Lee Mair gifted Doyle the ball inside the box with a weak clearance and the striker turned inside the centre-back to leave him facing the St Mirren goalkeeper 14 yards out, but Samson got down well to push wide.
Doherty fired an angled strike over the bar after finding space inside the box and Hibs were made to pay for the missed chances in the 65th minute.
Jeroen Tesselaar burst forward on to an instant and perfectly-weighted return pass from substitute Imrie
before laying the ball inside to McLean to sidefoot just inside the far post from 18 yards.
Hibs soon had a penalty claim rejected when substitute Imrie stole in between Griffiths and the ball as the striker attempted to shoot. Imrie took the ball away as Griffiths lay injured and Fenlon remonstrated with the officials.
Hibs, who had Roy O'Donovan on for Doyle, were given a lifeline when McAusland, who had been booked in the 36th minute for bringing down Isaiah Osbourne with a sliding challenge, was shown a second yellow card for pulling back Griffiths.
St Mirren had just brought on their final sub, but fortunately for them that man was centre-back Darren McGregor, who had replaced Jim Goodwin in midfield.
Hibs had brought on Sproule moments earlier and soon brought on midfielder David Wotherspoon for left-back Pa Kujabi.
But the visitors failed to test Samson further, with a spectacular but wayward scissors kick from Griffiths summing up their desperation.
Stevenson, booked earlier for a late tackle on Imrie, was then penalised for a challenge on McGowan to cap a frustrating afternoon for Hibs.
