Cleckheaton brushed aside Division Two leaders Undercliffe by eight wickets in their last-four clash, while Saints overcame East Bierley by three runs in a high-scoring thriller.
This set up a heavyweight clash between the sides currently lying first and third respectively in Division One.
The semi-final at South View Road kept a large crowd enthralled right up to the final ball.
St Lawrence won the toss and openers Adam Waite and Mark Robertshaw shared a stand of 190, Waite making 106 – including two sixes and ten fours – and Robertshaw 91.
Skipper James Smith then weighed in with 45 not out from 24 balls to push the total up to 283-4.
But East Bierley did not shrink from the challenge, with captain Gavin Hamilton (86) and Richard Atkins (78) leading the way.
They needed 14 to win from the last three balls and, after hitting a six and a four in successive deliveries, overseas player Mohammed Irshad was caught on the boundary by Steve Watts from the bowling of leg-spinner Tom Hudson (4-66) – to the delight of the St Lawrence supporters.
Undercliffe, who had beaten Division One sides Morley and Pudsey Congs on the way to the last four, saw their hopes of another upset disappear when they lost five wickets for no runs to plunge from 84-2 to 84-7 as left-arm spinner Andrew Deegan took 5-20.
A 108 total was never going to seriously tax Cleckheaton and they eased home after overseas player Tanzeel Altaf smashed a six and eight fours in a breezy 41, while opener Tim Jackson made 38 not out.
It will be Cleckheaton's second final – they lost by 51 runs to Pudsey Congs in 2007 – while Saints, who have been drawn away in every round, will be bidding for a sixth triumph
Meanwhile, Pudsey St Lawrence Seconds are through to the Sovereign Health Care Priestley Shield final.
Hanging Heaton defeated 2013 Division One champions Morley by six wickets, while Saints beat 2012 winners Keighley by 40 runs at Tofts Road.