Needing a win to qualify for the tournament proper, Mumbai failed to post an imposing total on Tuesday after being put in to bat, reaching 132-9 from their 20 overs.
And the Indians' bowling attack could do little to repair the damage as the Knights coasted to 133-4 with almost three overs to spare.
The six-wicket win secured their place in the main competition, alongside Lahore Lions.
Tim Southee (3-24) and Scott Styris (3-21) did the majority of the damage with the ball for New Zealand-based Knights, who had reduced Mumbai to 46-5 midway through their innings.
Battling performances from Kieron Pollard (31), Shreyas Gopal (24) and Lasith Malinga (20) did help lift the Indians but their total always looked likely to be short.
The Knights quickly set about chasing it down and a Kane Williamson half-century (53) and a strong showing from fellow opener Anton Devcich (39) lifted them into a position of dominance that they never relinquished.
There was also cause for celebration among the Lahore Lions players, who had earlier done their part in their qualification bid with a comfortable 55-run triumph over Southern Express.
Captain Mohammad Hafeez produced a man-of-the-match display for the Lions, with his 67 proving pivotal as they posted 164-6 despite the rest of the batting line-up struggling to make big scores.
Only Saad Nasim (31) and Ahmed Shezhad (29) were able to do any damage, but Hafeez led from the front to see his side through.
Express rarely looked like getting anywhere near that mark as they were bowled out for 109 with two overs to spare.
Aizaz Cheema (3-15) was the star of the Lahore bowling attack, taking two wickets in the fourth over to help the Lions blast their way through the Sri Lankan side, although he missed out on a hat-trick for the third time in the tournament.
The group stages of the competition start on Wednesday as Kolkata Knight Riders face Chennai Super Kings.