The winger entered the history books when a break down the left edge put him over for his 213th career try, and his second in Friday's clash with bitter rivals Sydney Roosters.
Johnston had earlier grabbed a Latrell Mitchell flick pass to score his first try of the night and equal Ken Irvine's mark of 212, which the legendary winger set in 1973.
Security presence was increased for the round-two clash after Souths players and coach Wayne Bennett had this week urged supporters not to invade the pitch.
But fans were undeterred by the prospect of $5,500 fines, having booed earlier in the game when the big screen displayed warnings against entering the field.
The majority took around five minutes to return to the stands as the ground announcer reminded the crowd that "we have a game of rugby league to play".
Prime Minister and devoted Souths fan Anthony Albanese was among those on the field celebrating rugby league history being made.
Players took refuge in the dressing sheds amid the chaos as a handful of the more sluggish supporters were detained by security and police.
Johnston applauded and waved to the crowd as the teams returned to the pitch, with the Rabbitohs receiving a warmer reception than their hosts.
Play resumed roughly 15 minutes after Johnston's try helped Souths level the scores at 12-12 and sparked the momentous scenes in the northern in-goal.
In commentary for Fox Sports, NRL legend Cooper Cronk called Johnston's feat "the greatest moment in our game's history for a long period of time".
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