Johnson missed three gilt-edged opportunities and had an 82nd minute goal disallowed for offside when he failed to look across the line.
Former Birmingham striker Johnson's first miss came on 20 minutes when Jobi McAnuff sent a long lofted ball over Danny Higginbotham. As Johnson advanced, Saints' rookie keeper Bartosz Białkowski failed to decide whether to come out or stay in his goal and the Palace forward's attempted lob drifted harmlessly wide.
Three minutes into the second half Michael Hughes sent a low left-footed cross into the path of Johnson and, despite getting ahead of Saints defender Darren Kenton, lifted his volley over the bar.
Johnson's hat-trick of misses was complete on 90 minutes with Hughes again the provider, sending a free-kick perfectly into the box but the Palace striker somehow headed wide from just eight yards out.
The home side's best chances came when either Marian Pahars or Andrew Surman were on the ball.
A 21st minute cross by Surman, fresh from an impressive loan spell at League One side Bournemouth, was somehow headed wide by Kenwyne Jones when he should have done better.
Four minutes before the break Pahars sent a crashing right-foot volley high to Gabor Kiraly's left but the Hungarian keeper did well to tip the ball over the bar.
In the 72nd minute Saints had a penalty appeal turned down when the willful Brett Ormerod got to the byline and crossed towards substitute Dexter Blackstock only for his intended pass to be handled by Palace defender Darren Ward.
The referee either failed to spot the incident or instead ruled it was ball-to-hand rather than a deliberate offence.
It was Pahars though who looked the most likely to break the deadlock and his run on 89 minutes lifted the home fans but his 20-yard left-foot drive was saved by Kiraly.