Still, Lancashire needed to score at nearly six an over in their chase, and a mix-up between Lewis and fellow opener Alice Clarke saw Clarke run out with only 20 on the board.
But Lewis and Smale broke the back of the innings with their composed partnership, although they survived a couple of close run-out calls before Smale fell to a good diving catch by Kemp at short mid-wicket off Mary Taylor.
And when Fi Morris was run out for nine, Lancashire were 183-3 with the required rate at more than seven-and-a-half.
However, captain Ellie Threlkeld added real impetus with 32 off 30 balls in a partnership of 60 with Lewis, until she fell to Freya Davies.
But as long as Lewis was at the crease, Lancashire remained favourites. She reached her century from 121 balls before going on the charge to record her highest List-A score.
In partnership with Ailsa Lister, they added an unbroken 46 off just 28 balls to see Lancashire home as they lifted the second trophy of the season to go with victory in the T20 County Cup in May.