Tom Webb picked up 4-36 as the visitors were restricted to 202 with Ben Quick top scoring on 48. Mason and fellow opener Lee Gordon (54) gave the home side a solid start and put them on the way to an upset.
Steeton hovered above the relegation spots last season but started with a victory over runners-up Bilton and added the full 10 bonus points to the 10 they get for winning under the scoring system introduced this year.
John Akrigg (63) and half-century makers Neil Spragg and Adam Foulds saw Steeton to 273-7 before Ross Suri (4-40) starred as the visitors were dismissed for 210, with Sajad Ali top scoring with 48.
Jonathan Hughes hit a century and with the support of Nick Cockcroft (55) and Dave Pennett (46) steered Ilkley to 297 and victory on their return to the top flight after 20 years’ absence. Half-centuries from Charlie Swallow and Bradley Ranford could only take Ben Rhydding to 205-7.
Harrison Durkin (76) and Brian Jackson (84) put on 131 and with 54 from Peter Towner, Guiseley set a 284-8 total which was too much for Collingham, despite the efforts of Stephen James, who followed up a four-wicket spell with 91.
Having restricted Rawdon to 227-8 in which Conor McInerney hit 61, Otley clinched victory with three balls to spare thanks to a six from James Wilcock.
Calum Lockett (81no) and Isaak Beck (50) led the way as promoted Upper Wharfedale started life in Division Two with a seven-wicket win at Green Lane.