Managed by Sunderland City Council, the park beat off tough competition to win the top prize.
The park was judged on criteria including environment, design, usability, access, maintenance and community involvement.
Mowbray Park's rich history, close links with the community and beautiful displays particularly impressed the judging panel, which included HW's deputy editor Matthew Appleby and gardening writer Peter Seabrook.
Seven parks across the UK were also recognised as regional winners in the annual awards - sponsored by lawnmower engine manufacturer Briggs & Stratton.
The winners were announced during a ceremony on Tuesday (9 December).
Queen's Gardens in Newcastle-under-Lyme topped the Midlands regional heat for its quality and the striking designs of its year-round floral displays.
In the South East, the 232ha of open space at Hylands Park in Chelmsford, Essex - which includes woodland and a Pleasure Gardens - was recognised as the region's best park.
Royal Parks-managed Regent's Park won London's central region category. Its spectacular horticultural displays and high levels of maintenance impressed judges.
In the South West, Weston-super-Mare's Grove Park was recognised for elements including its high levels of maintenance, safe atmosphere and sensory garden.
Cardiff's Bute Park won the top spot for Wales with its extensive historic parkland that acts as a "green lung" for the city.
Cumbria's Barrow Park, in Barrow-in-Furness, was awarded the North West prize, while Glasgow's Tollcross Park received the award for Scotland.
Last year, Glasgow's Pollok Country Park won Britain's Best Park 2007, before going on to fight against competition from green spaces in Italy, France, Germany, Poland and Sweden to be crowned Europe's Best Park.