The 16th-seeded Sock, 25, became the first American to win the tournament since Andre Agassi in 1999.
Sock, who started the week in 24th position in the Race to London, needed to win the title to leapfrog Spain's Pablo Carreno Busta into eighth place.
He did just that with a solid performance, wearing down the defence of world number 77 Krajinovic.
Sock won his first two service games to love but ran into a spot of bother in the sixth game. He served and volleyed to save a break point but a double fault handed Krajinovic a break and a 4-2 lead.
The American broke straight back but a fine forehand earned the Serb a set point and Sock made a mess of another forehand as his opponent, who defended well, took the lead.
Krajinovic, however, lost his focus and a forehand error gave Sock an early break in the second set. The Serb fell 4-1 down and could only break back once, Sock levelling for one-set all.
The momentum had shifted and Sock, who will finish the year as the American number one for the first time, raced through the deciding set to claim his maiden Masters title.
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