The 20-year-old midfielder was inspired by the Foundation's work with James Geddes, a next-door-neighbour of the Longstaff family in North Shields who was diagnosed with leukaemia as a teenager.
With support from United's official charity through its wellbeing work in the Great North Children's Hospital and employability programmes based at St. James' Park, James has regained his confidence and built lasting friendships with Foundation staff.
Longstaff's new role sees him joining his brother, teammate and fellow Foundation ambassador, Sean, in promoting the work of the charity and its aims of motivating and mentoring individuals to achieve their potential.
He hopes his involvement with the Foundation will encourage more children, young people, families and elderly adults to access support from the charity through its education, sport, employability and health and wellbeing programmes.
On becoming the latest Newcastle United Foundation ambassador, Longstaff said: "I'm really excited to become an ambassador for the Foundation because they do amazing work in the community and I'm very proud to be part of that.
"I've grown up here, so I know how important Newcastle United is to everyone and how passionate and tight-knit our communities are. The Foundation is a big part of that and can offer something for everyone and for people of all ages.
"I've seen myself how the Foundation has helped people like James to get through really challenging times and I think it's important for people to know there is always support out there if they need it."
Longstaff has been a long-standing supporter of the charity and also attended its most recent annual awards dinner to celebrate the hard work and perseverance of outstanding individuals supported by the charity.
In their ambassador roles, the Longstaff brothers join Foundation patrons Shola Ameobi, Jonathan Edwards CBE, Gabby Logan MBE and Bob Moncur in helping to promote the charity's work across the North East.
Steve Beharall, Head of Newcastle United Foundation and former coach to the Longstaff brothers at Newcastle United's Academy, said: "We're delighted to have Matty's support and look forward to working more closely with him to reach more people and communities across our region.
"Matty is a genuine role model for so many people we work who can be inspired by his journey as a young lad from North Shields to living his dream at his boyhood club.
"Matty and Sean are both smashing individuals and are genuinely committed to helping others, which makes them the perfect ambassadors for our charity and we're very proud to have them both on board."
Sean Longstaff became an ambassador for the Foundation two years ago, moved like his brother by the charity's support of next-door neighbour, James Geddes.
Both brothers are keen to offer particular support for chronically-ill young patients receiving care in Newcastle's Great North Children's Hospital.
Here, children and teenagers and their relatives are encouraged to play, learn, laugh and stay active with a range of activities delivered by the Foundation, with support from the Sir Bobby Robson Foundation, players of the People's Postcode Lottery and Great North Children's Hospital Foundation.
