Promoted Eastbourne Borough failed to clinch the eastern division title despite beating Fisher Athletic 2-0 in their final league game of the season.
Dorchester spoilt the Priory Lane promotion party by defeating King's Lynn 1-0 and taking the championship on goal difference.
But there was still plenty to celebrate for Borough who are set for premier division football after just three years in the eastern division.
Boss Garry Wilson said: "When you consider what Dorchester are paying out and what we are paying out, we are worlds apart.
"They went for that league title and paid a lot of money into their promotion effort but we have stayed with them all the way.
"We are now more weary than anything else and we'll just have to lift ourselves for the Senior Cup final today."
Assistant boss Nick Greenwood added: "We have exceded our wildest expectations and we are absolutely thrilled. The players have been fantastic."
Borough beat Fisher without ever getting into top gear and it was a drab affair for 937 spectators, the biggest league gate at Priory Lane this season.
Eastbourne made a bright start with a superb goal in the 14th minute.
Andy Ducille slipped a perfect ball out on the left for Matt Crabb who made ground before firing into a top corner.
It was all Borough for the remainder of the first half with John Westcott firing inches wide.
The game went to sleep after the interval but briefly came to life when home keeper Dean Lightwood superbly turned a Mark Parrish pile-driver over the bar.
Borough made the game safe in the 79th minute after Fisher keeper Michael Holder flapped at a Westcott cross under challenge from Scott Ramsay.
The former Albion striker pounced on the loose ball to bang in his 38th goal of the season.
After the game the eastern division runners-up cup and individual awards were presented to Borough as fans stayed behind to celebrate.
St Leonards' last match in the Dr Martens League ended in a 1-0 defeat at Fleet Town.
Saints will go back into the County League next season.
The decisive moment came when Ryan Luffman spun Paul Henty on halfway, breezed past Tristan Wood and fired past Greg Nessling ten minutes into the second half.
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