Trailing 5-3 from the afternoon, Selby made breaks of 85, 67, 86 and 90 on the way to establishing a 10-7 advantage.
Fellow Englishman Murphy went closest to the first century of the match with a run of 98 but otherwise struggled to find his rhythm as Selby dominated.
The best-of-35 final continues on Monday at 13:00 BST – live on BBC TV.
“I felt a bit edgy at the start, it’s a world final and I was trying that extra bit harder and Shaun started off a lot better than me,” Selby told BBC Sport.
“Every final I have been to I have always felt really tired going into it but I have dug in today, grafted when I needed to and manged to get myself a lead.”
Murphy, aiming to win his second world crown 16 years after his 2005 triumph – which would become the largest ever gap between titles – had eked out a slender lead on Sunday afternoon, in front of a crowd of around 600, the highest of the tournament so far.