David Clarke’s team wrapped up the crown on the final day of the season as they eased to a seven-wicket success over Wollaston, who had already been relegated to Division One.
Lewis Bruce’s spell of 5-23 ensured Wollaston were dismissed for just 109 with Matthew Jones (38) the only batsman to pass 20. And Peterborough cruised to victory on 113-3 in the 24th over with Josh Smith (41) and Clarke (34) leading the way.
Finedon Dolben had to settle for the runners-up spot with their final-day five-wicket success over Brigstock ensuring they finished 15 points behind the champions.
Overstone Park will be playing top-flight cricket next season as they were confirmed as Division One champions.
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The table-toppers wrapped up their season with a huge 150-run victory at Old Northamptonians 2nd.
Charles Edwards (167) and Vikesh Patel (110) put on a second-wicket stand of 271 to get Overstone to a big total of 340-3 before ONs were dismissed for 190, Premal Patel taking 4-41.
Burton Latimer finished as runners-up in Division One while Kettering and Earls Barton ended up in the relegation places.
There was better news for Finedon in Division Two as their second team were crowned as champions and they will be joined in Division One by Long Buckby, who finished just two points clear of third-placed St Crispin & Ryelands after both clubs won on the final day.
In Division Three, there was title glory for Wellingborough Indians as they racked up over 500 points.
They completed a superb campaign with a 22-run win at Irchester.
Indians put 249 on the board with Pritam Patel (63), Sagar Mangalorkar (60no) and Bhavin Patel (54) the top scorers and they then bowled the hosts out for 227, despite the efforts of Thomas Duxson (77) and Rishi Patel (48no).
NCL Ups and downs for 2019
Champions: Peterborough. Runners-up: Finedon Dolben. Relegated: Wollaston.
Champions: Overstone Park. Relegated: Earls Barton, Kettering.
Champions: Finedon Dolben 2nd. Promoted: Long Buckby. Relegated: Northampton Saints 2nd, Isham.
Champions: Wellingborough Indians. Promoted: Heyford. Relegated: Stony Stratford 2nd, Horton House 2nd.
Champions: Bowden. Promoted: Overstone Park 2nd. Relegated: Kempston, West Haddon & Guilsborough.
Champions: Spencer Bruerne. Promoted: Carrib United. Relegated: Stony Stratford 3rd, Northampton Saints 3rd.
Champions: Bugbrooke. Promoted: Loddington & Mawsley 2nd. Relegated: Isham 2nd, Podington 2nd, S&L Corby 2nd.
Champions: Hardingstone. Promoted: Mears Ashby, Earls Barton 2nd. Relegated: Gretton, Bugbrooke 2nd, Wellingborough Indians 2nd.
Champions: Heyford 2nd. Promoted: Barton Seagrave, Old 2nd. Relegated: Irchester 2nd, Long Buckby 2nd, Raunds 2nd.
Champions: Brigstock 2nd. Promoted: Great Houghton 2nd, Geddington 3rd. Relegated: Oundle 3rd, St Michaels (N), Brixworth 3rd.
Champions: Finedon Dolben 4th. Promoted: Abington Phoenix, Hardingstone 2nd. Relegated: Stony Stratford 4th, Bold Dragoon 2nd, Weekley & Warkton 3rd.
Champions: Grange Park. Promoted: Rushden & Higham 3rd, Obelisk 2nd. Relegated: Irthlingborough 3rd, St Crispin & Ryelands 4th, Old Northamptonians 5th.
Champions: Mears Ashby 2nd. Promoted: Kettering 4th, Geddington 4th. Relegated: Horton House 4th, S&L Corby 3rd, West Haddon & Guilsborough 2nd.
Champions: Willoughby 2nd. Promoted: Carrib United 2nd, Great Oakley 2nd.