Still reeling from John Aloisi's decision to stand down as coach on match eve, the Roar conceded the lead 10 minutes into the second half but appeared back in the contest via a controversial penalty with five minutes to play.
Reviewed by the VAR when forward Adam Taggart went down following minimal contact in the box from Aaron Calver, the Roar had the chance to level at 2-2 from the spot.
But Redmayne dived well to his left to deny Taggart's shot, helping hand Brisbane their equal club-worst fifth straight loss and a tough start to Darren Davies stint as interim manager.
Meanwhile the win was Sydney's third in a row, and momentarily moved them to second on the A-League ladder - albeit with pacesetters Perth still to play this weekend.
Le Fondre was again dangerous, scoring his eighth and ninth goals of the season to move three clear of his nearest rival for the league's golden boot award.
After both sides traded goals in the first half, the Englishman's second in the 55th minute gave Sydney control of the match off the back of wonderful lead-up play from Milos Ninkovic and Siem de Jong.
The pair helped Brandon O'Neill into space down the left edge, before the midfielder crossed it low to a running Le Fondre, who tapped it past goalkeeper Jamie Young with his non-favoured left foot.
The 32-year-old's first had also come from a nice lead up, this time in the 16th minute when defender Michael Zullo provided a perfectly-placed deep cross from well outside the box.
Le Fondre sprung free and ran onto the ball with virtually no opposition, allowing him to head home.
However, the Roar were back level in the 35th minute when the VAR ruled Matthew McKay had not handballed as he claimed Brisbane's first goal of the night.
After Eric Bautheac crossed into the box, Dane Ingham had Redmayne fending the ball away onto McKay's upper left chest, before the midfielder put it into the back of the net.
But the lead was only short-lived, as Sydney's 58 per cent share of possession eventually told in a well controlled second half.
