A three-time runner up, including demoralising crashes in the final races of the year in both 2010 and 2012, Power overcame mental exhaustion and closest rival - Team Penske team-mate Helio Castroneves - to finish the 250-lap MAVTV 500 at Auto Club Speedway in ninth place to seal the title.
The race was won by Tony Kanaan with Ganassi team-mate Scott Dixon finishing in second ahead of Ed Carpenter, who overcame an earlier pit-lane penalty to earn a spot on the podium.
But the spotlight was on Power after the race with the 33-year-old becoming the first Australian since former Formula One driver Alan Jones in 1980 to win a major international motorsport championship.
"That was one of the hardest races ever. Oh my god. I was crying over the line," Power said after the race.
"I'm so mentally exhausted now. My hands are numb from holding on to the wheel so tight.
"I want to be a lot more excited but I'm so drained right now. I can't believe I've won it."
Holding a 50-point lead going into the event, Power made a slow start to the race after qualifying 20th, entering the top 10 for the first time after 74 laps before doing well to move into seventh just after the halfway stage.
A yellow flag after Ryan Hunter-Reay spun out with 70 laps remaining saw the race restarted, giving Power the chance to further cut the gap.
When Castroneves was given a drive-through penalty for a pit lane infringement with 30 laps remaining, it was just a matter of Power getting through the race unscathed
Power did just that, crossing the line in ninth place to ensure he finally claimed the trophy he so desperately craved.