Josh Inglis was dropped three times on the way to posting 138 off 110 balls in Wednesday's one-sided clash.
Cameron Bancroft (90 off 112) and Mitch Marsh (56 off 30) contributed important knocks as WA posted a mammoth 7-387.
It equalled WA's previous best of 387 set against Queensland in 2018.
In reply, SA moved to 0-84 courtesy of strong knocks from Henry Hunt (50) and Kelvin Smith (44).
But the wheels fell of quickly from that point on, with the Redbacks losing 10-122 on the way to being dismissed for 206 in 31.4 overs.
Spinner Agar was the chief destroyer with figures of 5-64, while AJ Tye chipped in with 3-34 as the Warriors claimed their 16th one-day title - a record in Australian cricket.
WA Cricket won the treble last season - the BBL title, the one-day cup crown and the Sheffield Shield.
They have the chance to repeat the dose this summer, having already taken out the BBL and one-day titles, as well as securing hosting rights for the Shield final.
"We speak about it a lot that we don't just want to win the odd title here or there, we want to start a dynasty and win everything," Inglis said.
"We have the squad to do it. We go out every single game expecting to win."
The Redbacks only had themselves to blame for allowing WA to reach such a big total in Wednesday's decider.
Inglis was a sluggish 26 off 42 balls with the scoreboard reading 1-55 in the 14th over when he was dropped by Hunt at deep mid-wicket.
The moment proved decisive, with Inglis needing just 45 more balls to race from 26 to his maiden one-day Cup century.
Inglis was dropped again on 118 when Redbacks captain Jake Lehmann grassed him at mid-off.
The 28-year-old blaster was given another reprieve on 123 when he gloved paceman Spencer Johnson through to the keeper - only for the umpire to give it not out.
Bancroft was on 51 when Henry Thornton dropped an absolute sitter at mid-off, with the ball going straight through his hands and to the boundary.
"Ultimately we need to be better. It is as simple and as brutal as that. It's not good enough," Redbacks coach Jason Gillespie said of the dropped catches.
"The lads are working at it, but we probably need to do more. We just can't keep giving opportunities to the opposition.
"They were chances that professional cricketers should take. That's what the most frustrating thing is."
The Redbacks' misfortune began even before play started with opener Jake Carder a late scratching after injuring his hamstring in the warm-up.
Smith was brought in as his replacement.
The 227-run stand between Inglis and Bancroft was WA's highest ever second-wicket stand.
Inglis cracked 14 fours and five sixes on the way to the highest score in a one-day cup final, beating the 135no from Michael Bevan in 2001 when NSW beat WA.
