Earlier, South Africa batsman Quinton de Kock scored 81 while India's Rohit Sharma made 47 to help three-times champions Mumbai post 187-5 in 20 overs.
"I'm playing the best cricket I've played over a prolonged period of time. I was disappointed to get out when I did... we'll take a lot of confidence from this win," man of the match Buttler, who has three half-centuries this season, said.
The 28-year-old has enjoyed his fine IPL campaign after also hammering a career-high 150 off 77 balls in the fourth ODI against the West Indies during England's recent tour of the Caribbean.
World Cup hosts England are set to name the in-form Buttler among their provisional squad for the tournament next week. They begin their campaign against South Africa at The Oval on May 30.
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Ian Chadband)
