Johnston's second try came just after halftime at Allianz Stadium and triggered an invasion of the pitch by fans, including Prime Minister Anthony Albanese - a lifelong Rabbitohs supporter.
The 31-year-old Johnston had equalled Ken Irvine’s long-standing record of 212 career tries ten minutes before the break with a typically cool finish in the left corner.
Many fans in the sellout crowd had yet to take their seats for the second half when centre Latrell Mitchell made a break through midfield and released Johnston to score his 213th.
Despite threats on the big screen of fines of $5,500 and a ban from NRL matches for two years, thousands of fans flooded on the pitch and mobbed Johnston.
Johnston ended up on the losing side as the Roosters took local bragging rights with a 26-18 victory.
“On behalf of everyone involved in our game, I congratulate Alex on an achievement that will stand the test of time,” NRL chief Andrew Abdo said.
“Ken Irvine was small in stature but a giant of Rugby League — one of the greatest wingers the game has ever known. His try‑scoring record stood for more than half a century and was considered by many to be untouchable.
“That makes Alex’s achievement truly extraordinary. He is one of the finest finishers we’ve ever seen, and it is fitting that he now stands alone as the Premiership’s greatest try‑scorer.”