
With Leiston's match being postponed on Saturday, Enfield's 3-0 win at Rookery Hill saw them go top of the league, meaning Leiston went into the rearranged fixture knowing a win was required to secure the league title.
There was never any concern for the home side as Danny Smy gave the champions an early lead, before Joe Jefford scored a second shortly before half time.
Midfielder Leon Ottley-Gooch scored a seven minute hat-trick in the second half, and Joe Jefford sealed the win with his side's sixth goal of the game.
The win sees Leiston follow in the footsteps of Lowestoft Town in winning the Isthmian League Division One North at the first attempt following promotion from the Eastern Counties Premier Division.
Blues now head to the Queen Elizabeth II Stadium on Thursday evening with kick off at 19:45.