Kerr-Barlow double takes La Rochelle into Champions Cup semis
Kerr-Barlow, 32, crossed either side of the interval to book the French side's place against Exeter on April 30 in Bordeaux.
In the other semi-final 24 hours earlier, record five-time winners Toulouse will face Leinster, champions on four occasions, at Lansdowne Road, the same venue as May 20's final.
At a vocal Stade Marcel-Deflandre, the hosts led 6-3 after 20 minutes as Owen Farrell responded to Antoine Hastoy's two early penalties.
The away side's England No. 8 Billy Vunipola left the field after 22 minutes and was seen with strapping on his right knee on the touchline holding crutches.
Sarries' poor discipline proved costly, conceding 10 penalties in the first half, as Hastoy added a third shot at goal after 25 minutes.
Kerr-Barlow's key intervention then came just after the half hour mark as he looped around No. 8 Gregory Alldritt.
He then spotted Saracens centre Nick Tompkins' poor defensive read on the 22m line and glided over to cross for his second try in as many games for the club coached by ex-Ireland fly-half Ronan O'Gara.
Hastoy kicked the extras to make it 16-3 at the break in spring sunshine on the Atlantic coast.
Kerr-Barlow, who also qualifies for Australia, highlighted his Rugby World Cup-winning quality just before the hour mark with his second try of the game.
Flanker Levani Botia, who can feature at centre too, made a break from a dummy pass and Kerr-Barlow picked a smart support line to score and the holders led 21-3.
With a quarter of an hour to play the Premiership leaders cut the gap to 21-10 as man of the match Botia's compatriot in prop Eroni Mawi bundled over.
The visitors' hopes of reaching a first semi-final since 2020 and their Premiership relegation for salary cap irregularities were ended five minutes later as Hastoy slotted his fourth penalty of the match.
Kerr-Barlow was replaced with 10 minutes left to a standing ovation from a sell-out crowd as did Will Skelton but the Wallabies lock was stretchered off with a worrying neck injury.
With less than two minutes to play the home side's France No. 8 Yoan Tanga was denied a try after a knock-on in a preceding line-out but it failed to dampen the home supporters' celebrations.