Steyn last made a Test appearance for the Springboks back in 2016 but the 37-year-old came on from the bench to prove the difference in Cape Town.
Having replaced Handre Pollard, whose kicking was off in the second half, Steyn converted two penalties to secure a hard-fought victory from a tense tussle.
After Steyn's first effort, Finn Russell's penalty seemed to have sent the series to a draw, only for Steyn’s boot to once again prove the Lions' downfall with a 79th-minute three-pointer.
The Lions seemed to have been handed an early blow when Dan Biggar was forced off injured after attempting to tackle Lukhanyo Am, wth Pollard converting South Africa's resulting penalty.
However, Biggar's replacement Russell kicked a penalty five minutes after his introduction to restore parity as he asserted himself on proceedings.
Russell was kicking again soon after, this time adding the extras after Ken Owens marked his maiden Lions Test start by bundling over at the back of the maul for the first try.
A second Pollard penalty reduced the deficit, while Russell had a fortunate escape when he made direct contact with Cheslin Kolbe's head, a slip from the South Africa winger helping the Lions remain at full strength.
Pollard's penalty dropped just short, yet the Springboks finally found the breakthrough moments later, Kolbe holding off Luke Cowan-Dickie's challenge to race over in the corner after a loose ball had broken in South Africa's favour – the try standing after a long TMO (television match official) review for a possible knock-on from Jasper Wiese.
An Ali Price error handed South Africa a chance to extend its lead, though the Lions held firm and Russell kept his cool at the other end to arrow a three-pointer through the posts after Eben Etzebeth's mistake.
Steyn, South Africa's hero in the 2009 series, entered the fray with 16 minutes remaining, and his first act was to nose the hosts back ahead.
Am's foul on Conor Murray handed Russell the chance to level the scores, an opportunity which the Scot took coolly, but it was Steyn who had the final say.
Herschel Jantjies tried to run the ball after the Boks were awarded a penalty 40 metres out, though fortunately the referee called it back, with Steyn holding his nerve to down the Lions.