Smith, making his first club appearance of the season after touring Australia with the British and Irish Lions in the summer, needed less than nine minutes to make an impact.
After Saracens failed to cleanly gather Smith’s high kick into their defensive 22, Quins eventually worked the ball to their talismanic number 10, who burst through two tackles to touch down in the corner.
Quins pressed to extend their advantage, but it was Saracens who scored next with a sensational team move started by Farrell’s excellent short pass to Lucio Cinti, who broke clear and set up fly-half Burke to stroll over unopposed.
Smith, denied a second try following intervention by the TMO because of a scrum infraction, kicked Quins three points ahead after Saracens were penalised at the reset scrum.
Almost immediately after the restart, home full-back Green contested another high Smith kick, booted the loose ball ahead, and won the foot race to ground the ball near the posts to make it 17-7 after Smith’s conversion.
A spell of Saracens attacking pressure ended with Farrell’s kick across field picking out the unmarked England and Lions back row Earl to touch down, again bringing the gap between the sides to three points with Farrell kicking the extras.
But a comeback from the visitors failed to materialise in the final 15 minutes, and after Quins debutant Boris Wenger had a try ruled out by the TMO for a double movement, Smith sealed victory by knocking over another penalty with the clock past 80 minutes.
Before kick-off, there was a minute’s applause in memory of former Harlequins academy coach Warren Abrahams, who has died aged 43.
“Coming in this morning, I heard the news about Warren,” Smith told TNT Sports.
“He coached a lot of us coming through the academy and this one meant a lot to us. May his soul rest in peace.”