The 16th-seeded Sock became the first American to win the singles title at 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 to ensure he will play in the Nov. 12-19 event in London.
He did just that with a solid performance, wearing down the defence of world number 77 Krajinovic.
"I obviously snuck into ... London unexpectedly so zero pressure on me to go there," said Sock, the first American man to make it into the singles draw of the season-finale since Mardy Fish in 2011.
"I'm going to swing big and play my game, like I always do, and just honestly enjoy the moment."
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 behind 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.
Despite the defeat, Krajinovic will climb to a career-high 33 in the rankings on Monday and the 25-year-old is aiming higher.
"I know that I will work even more right now. That's for sure. Pressure is part of the sport," he said.
"Everybody has pressure in life. Of course, I am going to have more. People will expect more from me right now. They see I can do it.
"But if I want to be better, I have to handle that. So I will work on that also. So I hope next year will be better."
(Reporting by Julien Pretot,; Editing by Pritha Sarkar)
