The Edinburgh team's fate was left in the hands of others when their clash with Aberdeenshire was abandoned after Myreside was ruled unplayable.
It meant 'Sonians would be denied their first top-flight title if second-placed Grange managed to beat outgoing SNCL champions Carlton at Grange Loan.
However, Grange were unable to seize their chance, going down by five wickets to spark celebrations across the capital.
Those celebrations were of the non-alcoholic variety for the Watsonians players who could today complete a the double by winning the Scottish Cup – also for the first time in their history.
Skipper Stuart Chalmers said: "When our game was off all we could do was wait to hear what was happening at Carlton.
"Luckily, it has gone our way and, although we didn't play today, I think we have earned the title over the season. We won some really close games early in the season and we're delighted."
While Watsonians celebrated, Grange were left to rue a missed chance after they could muster just 167-8 against Carlton.
Ryan Flannigan, ironically a former Watsonians player, led the way with a battling half-century.
However, his 69 came from a painstaking 115 balls and included just one boundary as a slow wicket and outfield took its toll. It was a measure of how hard the Saltires man had to work that none of his team-mates reached 30 runs.
Omar Ahmad was Carlton's most successful bowler, dismissing Gregor Maiden with his first delivery and going on to claim a further wicket at a cost of just 14 runs from his nine overs.
Grange, though, fought back well in the field and were firmly in contention when Andrew Brock took two wickets in as many balls to reduce the hosts to 84-4 in the 25th over.
However, Preston Mommsen showed his class with a fine unbeaten half-century while former Northants player Toby Bailey contributed 34 to a fifth wicket stand of 68 to put Carlton in control. The home side eventually reached their target with more than three overs to spare, Mommsen hitting the winning run to bring up his 50.
Elsewhere in the east, Saltires prospect Michael Leask scored an astonishing 168 as Stoneywood-Dyce beat relegated Forfarshire at Forthill.
Meanwhile, there was jubilation for Dumfries who won the Western Premier Division at the first time of asking. The Nunholm men went into their clash with West of Scotland knowing victory would be enough. Equally, defeat could have brought both West and Clydesdale into the equation.
However, Dumfries racked up 196-7 before bowling out their rivals for 134.
Clydesdale's seven-wicket win over Greenock at Titwood became academic except for the visitors, who are relegated, while Stirling live to fight another day.