Akpo Sodje struck his third goal in three games after 26 minutes as Hibs took the lead at Easter Road.
However, the Buddies were awarded a soft penalty after 39 minutes when Francis Dickoh was adjudged to have fouled Paul McGowan and Craig Dargo netted the subsequent spot kick.
Hibs now face an uphill struggle to finish top of the bottom half of the table, and sit eighth, one point behind Inverness having played two games more.
St Mirren began the month with back-to-back wins for the first time this season before losses to either half of the title-chasing Old Firm.
Despite the shock of being on the wrong end of Hamilton's first win since October, Hibs started the better, without either side making the goalkeeper work in the opening quarter.
Victor Palsson was booked for a lunge on Jim Goodwin in the centre circle after 24 minutes, a challenge which angered the Buddies midfielder and continued the duo's running feud which began in Hibs' February 2 win at Easter Road.
Palsson was soon smiling when Hibs struck on the break after 26 minutes to take the lead.
Derek Riordan played through strike partner Sodje to tuck into the bottom right corner.
It was the striker's third goal in three games and fifth since moving to Easter Road from Charlton in January.
Riordan then combined neatly with Callum Booth to tee-up Liam Miller, who should have done better than fire over from 12 yards.
Saints levelled in somewhat fortuitous fashion six minutes before the interval.
An innocuous-looking challenge by Dickoh on McGowan resulted in referee Alan Muir pointing to the spot and Dargo converted the
Palsson was replaced by Ian Murray after 56 minutes and could not resist a parting word with Goodwin - justifying manager Colin Calderwood's change for fear of being reduced to 10 men.
After Sodje evaded Darren McGregor on the right side of Saints' box but could not find a clinical touch, Goodwin found himself booked for felling Miller.
Calderwood had an animated conversation with fourth official John McKendrick soon afterwards, at one point throwing his notes down in disgust inside his technical area.
Lewis Stevenson replaced David Wotherspoon and Ricardo Vaz Te came on for Miller as Calderwood sought fresh impetus which never came.
McGregor was cautioned for a foul on Sodje, who recovered to hit the post with a first-time volley from Booth's free-kick, but the flag was already raised for offside.
Dargo was replaced by Gareth Wardlaw with six minutes remaining and the striker immediately called Brown into action, only to see his header from Kenny McLean's corner held.
Steven Thomson was booked for tussling with Ian Murray before St Mirren replaced Hugh Murray with David Barron as the visitors held on.