Buttler's England team mate Ben Stokes and Sussex all-rounder Jofra Archer took two wickets apiece as the Royals restricted holders Mumbai to 168 for six in their 20 overs.
The Royals then put in a solid batting display, led by Buttler, and notched 171-3 in 18 overs to win their third consecutive match and keep their playoff hopes alive.
"I'm really enjoying it, found some good form, it's a do-or-die situation and I'm full of confidence so want to keep it going," Buttler said at his player of the match award ceremony.
"I've spent a lot of time batting in the middle order so if I can bat through the first six overs I can tie the two together."
Buttler, who helped Mumbai win last year's title, admitted that his familiarity with Wankhede Stadium had helped his cause.
"Nice to come back here, I enjoyed my time at Mumbai and it properly helped to know the conditions. I'm in a good head-space and looking forward to the next game."
(Reporting by Aditi Prakash in Bengaluru; Editing by John O'Brien)
