United legend Beckham was joined by former team-mates Phil Neville and Gary Neville, Ryan Giggs and Nicky Butt in the stands at Wembley to watch Salford reach League Two for the first time.
Graham Alexander's side took the lead through Emmanuel Dieseruvwe and Carl Piergianni's second-half header was followed by Ibou Touray's decisive third.
Salford, who finished the regular season in third place in the fifth tier, beat Eastleigh on penalties to book their place in the final.
They have now achieved four promotions in five seasons since the Nevilles, Butt, Giggs and Paul Scholes took a 60 per cent stake in 2015 in conjunction with a 40 per cent share owned by their associate Peter Lim.
When Scholes had to give up his stake during a brief spell as Oldham manager this season, former England captain Beckham took on his old team-mate's share in Salford -- a club based just 3.5 miles from United's Old Trafford stadium.