Smith ended on 63 not out off 47 balls with four fours and three sixes as Mumbai reached 141 for three, in reply to the Lions’ 140 for five, to clinch victory with nine balls of their innings remaining.
The win kept Mumbai in the hunt for a semifinal place while the Lions, baring a mathematical miracle, are out of the running with a match to play.
Opening batsman Smith played a measured innings as he powered his way to his fifty off 33 deliveries while he shared in a 43-run third-wicket stand with Rohit Sharma (20) and a 51-run unbroken fourth-wicket partnership with Kieron Pollard.
Pollard was at his belligerent best as he hammered his way to 31 not out off a mere 20 balls with his knock including two fours and three sixes.
Mumbai had made a slow start to their run-chase losing the wicket of Sachin Tendulkar (5) and Dinesh Karthik (13) to slip to 47 for two in the eighth over.
But Smith blunted any hopes that the Lions’ had of winning the match before he and Pollard ended the match in a hurry with their partnership using up just 5.5 overs.
Mumbai had won the toss and asked the Lions to bat on a well-grassed pitch which offered up decent bounce and carry for the fast men.
But the Lions’ batsmen were unable to build on reasonable starts with Rassie van der Dussen (13), Quinton de Kock (19), Neil McKenzie (15) and Jean Symes (14) all reaching double figures without going on to put together a match-winning innings.
Alviro Petersen (35 not out) and Dwaine Pretorius (31 not out) finally provided the innings with some late impetus with a sixth-wicket partnership which realised 59 runs off 6.4 overs.
Left-arm spinner Pragyan Ojha ended with figures of 2-26.