Kelsey Mooney handed the Pilgrims a 26th minute lead to bag his third goal in his last two friendly appearances.
Stamford skipper Rob Morgan equalised on the hour mark for Graham Drury’s side.
United (4-1-2-1-2): Cameron Gregory; Zak Mills, Michael Bostwick, Tom Leak, Brad Nicholson; Martin Woods; Jordan Richards (Tom Edge 73), Keziah Martin (Sam McLintock 59); Keaton Ward; Kelsey Mooney, Michael Gyasi (Oliver Wilson 59). Subs (not used): Dylan Hill, Sam Essien, Will Hall, Tyler Knowles.
Referee: Lee Hartley.
Attendance: 388.
PIC: Rob O’Brien.