Mowbray may have ended the regular season rock bottom, but the final day belonged to their captain Luke Scott.
With a maiden century an elusive 10 runs away overnight, Scott surged into double figures and did not look back, doubling his previous highest score with an unbeaten 178 which secured victory and ruined any hope South Launceston had of making finals.
A healthy haul of 22 boundaries and a six went into the fabulous five-hour knock which took the Eagles to 7-268 in what is likely to be James Storay's last game as coach.
The Knights, for whom Jeremy Jackson took 3-74 and Brodie Jarrad 2-53, responded with 213 all out inside 50 overs as last year's grand finalists were unable to dislodge Riverside from third spot on the ladder.
Storay finished his tenure in charge at Invermay Park in style with figures of 5-65 plus three maidens.
Spencer Hayes also took 3-42 as Sisitha Jayasinghe (54) and Doug Ryan (38) led the Knights' run chase.
Opening batsman Matthew Battle was the day's other unbeaten century-maker as Westbury inflicted Launceston's first league loss of the season.
The result at Ingamells Oval means the second-placed Shamrocks will host third-placed Riverside coming off the bye next week for the right to face the ladder-leading Lions in the grand final.
Launceston declared on 8-204 as Sam Elliston-Buckley (37), Rowan Smith (33), George Maguire (30), Tom Gray (28*), Cam Lynch (26) and Dilan Sandagirigoda (21) shared the runs around while Kieren Hume (2-44) and Jonathon Chapman (2-50) did likewise with the wickets.
But Battle lived up to his name with a gritty 108 not out off 143 deliveries which included 16 fours and two sixes.
Nathan Parkin (47) provided the most support in a 128-run first-wicket stand while Jordan French was also pivotal with a run-a-ball 30 in the unbeaten fourth-wicket partnership.
Spinner Ben Humphrey took 2-35 for Alistair Taylor's men but couldn't prevent Westbury claiming the first-innings win at 3-207 in the 47th over.
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