The 19-year-old kept plugging away in blustery conditions to land four penalties from eight attempts, including the decisive kick after Gloucester had snatched a late lead.
It was Farrell's charged-down drop-goal attempt that led to Nicky Robinson's breakaway try for the Cherry and Whites - but he then stepped up to seal Saracens' 12-10 victory.
While Farrell scored the points, Saracens' defensive performance was led by another lung-bursting contribution from Namibian flanker Jacques Burger.
Saracens' 12th consecutive victory teed up a final against Leicester at Twickenham on May 28, a repeat of last year's showpiece.
"It says everything about Owen Farrell that he missed a couple of kicks at goal and he is the guy who wants to put his hand up to kick the winning penalty," said McCall.
"He has been incredible for us over a six-month period. He has guided the team to this run of victories we have had and he did that again today.
"We felt we were in control in the first half but it wasn't evident on the scoreboard. We left some chances out there. It was tight and nervy.
"Jacques embodies everything we are about in terms of physicality, work rate and discipline. I thought he was extraordinary today, especially in those last 15 minutes. I don't know how many tackles he made but it was a lot."
Gloucester's try came when Saracens were down to 14 men following a controversial decision from touch-judge Peter Huckle that led to Steve Borthwick being sin-binned for challenging Alex Brown in the air.
It has been a weekend of controversial calls from the referees' assistants following yesterday's decision at Leicester not to send off Manu Tuilagi for punching Chris Ashton.
McCall did not want to comment on the general issue of officials but he did say: "Everybody knows the touch-judge made a big error (over Borthwick).
"We expect better at this level in these kind of games, it could have been a match-changing error for us to go down to 14 men with 16 minutes to go.
"They scored their try and it could have been match-changing. I am pleased our boys had the resolve to come back and seal the victory."
Gloucester boss Bryan Redpath never felt confident his side would be able to close out the victory after Robinson's try.
But he was proud of what they have achieved this year in reaching the semi-finals and winning the LV= Cup.
"We didn't perform very well and I was never relaxed about it at the end," said Redpath.
"That is what pressure is, that is what top-flight rugby is. We just weren't good enough in certain parts today. We will have to take it on the chin.
"In 12 months they have come from a position where people wouldn't even have given them a sniff in the top six, to finish in third and come to Saracens and not let them score."
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