The league leaders saw off near neighbours Blackpool Wren Rovers 1-0, meaning Fulwood Amateurs had to match their result to keep the title race alive.
But Tony Hesketh's men fell to a 2-0 defeat at Tempest United to spark wild celebrations among the Thornton contingent.
Elsewhere, Whitehaven Amateurs secured their Premier Division status with a 1-0 win over Burscough Richmond.
In Division One, Horwich St Mary's got the win they needed to stay up, seeing off Hawcoat Park 3-1.
*We'll have more on Thornton's success in the coming days.
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