Royals failed to hit top gear in a must-win clash at St Mary's as the pressure mounted on McDermott and his men following a fourth straight Premier League defeat.
Jason Puncheon's second-half strike was enough to give Saints a deserved victory in the basement battle and leave Royals deeply entrenched in the relegation zone.
And Reading manager Brian McDermott admitted his side's performance, particularly in the second half, was simply not good enough.
"We didn't do ourselves justice in the second half," said McDermott. "We have to be better than that, we need to combine a performance in all areas, everything needs to come together. It was the poorest we've played for a while. We've got to pick ourselves up."
In a game of few clear-cut opportunities at St Mary's, it was the hosts who went closest in the opening period through Morgan Schneiderlin and Puncheon.
Schneiderlin failed to hit the target from six yards out while Puncheon had the ball in the back of the net from a corner, only for the referee to rule it out for a dubious foul on Royals keeper Adam Federici.
If that decision left Saints frustrated it could have been even worse as Reading winger Hal Robson-Kanu headed against the post in first-half stoppage time.
But the hosts raised their game after the interval and barely allowed Royals a sniff at goal.
Puncheon forced Federici into a smart save from the edge of the box before giving his side the lead on the hour mark.
Nathaniel Clyne was allowed space to run down the right and he played in Puncheon, who drilled a low shot into the far corner of the net.
Royals were forced into a change shortly afterwards due to an injury to Sean Morrison, but even Alex Pearce's long-awaited return could not spark the visitors into life.
Instead it was Saints who threatened a second, with substitute Guly Do Prado curling an effort on to the roof of the net and then forcing a sharp save from Federici.
Reading did threaten to snatch an equaliser when Adam Le Fondre headed wide from Robson-Kanu's centre.
But Saints took the honours to move out of the bottom three, while Reading were left to quickly recover as they prepared for another crucial six-pointer away at Sunderland on Tuesday night (7.45pm ko).
McDermott suggested: "I thought we'd really step it up in the second half but we didn't do that in the way we needed to. We had a couple of little chances, Adam Le Fondre with a header and then Jobi McAnuff shot over the bar.
"To only have that is very unusual for us. Anyone who has seen us over a long period of time will know we've always reacted well in games. Normally we throw the kitchen sink at them but we didn't on Saturday.
"We're obviously disappointed, we all are after a game like that. We know the league we're in, we've got to keep our heads down, dig in and get on with it."
Royals boss also sympathised with the 2,372 supporters who made the trip to the south coast: "I feel sorry for our fans, they've gone to Southampton and we haven't got a result. We play for them and they'll be as frustrated as we are, everyone in the dressing room is bitterly disappointed."
Southampton: Davis - Clyne, Yoshida, Fonte, Shaw, Schneiderlin, Cork, Puncheon, Lallana (c) (Ramirez 40), Rodriguez (Do Prado 79), Lambert. Subs not used: Boruc, Hooiveld, Ward-Prowse, Davis, Mayuka.
Reading: Federici - Cummings, Shorey, Mariappa, Morrison (Pearce 65), Leigertwood, Tabb (Hunt 69), McAnuff (c), Robson-Kanu (McCleary 77), Roberts, Le Fondre. Subs not used: Taylor, Harte, Guthrie, Samuel.
Attendance: 29,331 (2,372 Reading).
Referee: Jon Moss (West Yorkshire).