The 23-year-old from Banbridge clocked 15 minutes, 50.55 seconds to take almost three seconds off Duffy's 1996 mark of 15:53.47.
Mitchell was third in the British Milers Club event won by Louise Small of England in 15:42.47.
Irish Olympian Kerry O'Flaherty finished seventh in 16:32.56.
O'Flaherty said after the race that she sustained a grade two calf tear at the 1800m mark which required hospital treatment.
The injury could keep the Newcastle women out for a number of weeks.
Mitchell's time was over a minute inside her previous personal best for the distance.
The QUB athlete will now attempt to achieve Athletics Northern Ireland's Commonwealth Games consideration time of 15:39.00 before the 30 September deadline.
Meanwhile, Northern Ireland Commonwealth Games sprinter Leon Reid showed impressive early-season form as he clocked wind-assisted 100m times of 10.26 and 10.27 in Waterford on Saturday.
The 22-year-old, whose mother was born in Belfast, is hoping to switch international eligibility from Great Britain to Ireland and has joined Wexford club Menapians.
Reid won a European junior 200m silver medal for Britain in 2013 and also finished second in the same event at the European under-23s two years ago.

The 22-year-old played in 51 games last season for the Saddlers, who suffered a 6-1 aggregate defeat by Barnsley in the League One play-off semi-finals.
He becomes Alan Stubbs' first signing being named Millers boss on 1 June.
Former Republic of Ireland Under-21 international Forde joined Walsall from Wolves in 2014, and has also previously played for Scunthorpe United.
Meanwhile, Stubbs' former assistant at Hibernian, Andy Holden, has joined Rotherham as first-team coach.
Find all the latest football transfers on our dedicated page.

The Scot, 30, was set to be in the British line-up to defend the title in Brazil but has been ruled out after his top two rides picked up injuries.
Hello Sanctos and Hello M'Lady picked up minor injuries at the start of the season and will not be fit for Rio.
Brash was the top-ranked showjumper in the world last year and is still GB's leading rider in the global standings.

Peter O'Brien, a 51-year-old married father of six, from Llanishen, Cardiff, and Mark Sim, 41, from Caldicot, Monmouthshire, were killed in the blast in the basement of the rod and bar mill at Celsa Steel in Splott.
Five men were also injured.
The company is working with the Health and Safety Executive (HSE) and police to determine the cause of the blast.
Search dogs were seen being taken to the site on Thursday while specialist search teams have also been involved.
The alarm was raised at about 10:30 GMT on Wednesday and dozens of emergency services vehicles were sent to the scene.
Firefighters tackled an "extremely difficult" fire in the basement of the building, where five people were injured.
One was treated at the scene and four were taken to the University Hospital of Wales, Cardiff.
Three of them have since been discharged.
Queen's athlete Emma Mitchell broke Teresa Duffy's 21-year-old Northern Ireland 5,000m record at a meeting in Solihull on Saturday night.
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Championship side Rotherham have signed Walsall winger Anthony Forde on a three-year deal for an undisclosed fee.
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Scott Brash, part of Team GB's Olympic gold-medal winning showjumping team at London 2012, will miss the Rio Games.
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Searches are continuing for the bodies of two men who died in an explosion at a Cardiff steelworks on Wednesday.