Having been beaten 2-0 at home by Lynn on Boxing Day courtesy of two late goals, the Linnets were in control earlier in this contest - with Olly Scott opening the scoring in the 11th minute and Adam Crowther doubling the lead seven minutes before the break.
Mitford emerged as a second half substitute and reduced the arrears six minutes into added time - after Aaron Jones had been sent off for the title-chasing hosts.
Newcomer Mitford was included on the bench, as Ian Culverhouse handed starts to Jake Wright and Harry Lewis in two changes from the Boxing Day encounter.
Lynn took the lead with the first move of real note - with SCOTT angling a drive across Sam Long and into the net off the far post.
United did their best to get back into the contest - but failed to seriously test Paul Jones in the home goal.
The Linnets doubled their lead seven minutes before half-time, when CROWTHER glanced home Michael Clunan's corner delivery.
The home side lost the services of wing-back Jones with 18 minutes to play following an altercation with Wright.
The Pilgrims reduced the arrears in the sixth minute of stoppage time when Jordan Burrow flicked MITFORD into the clear - and the debutant beat keeper Jones with aplomb.
King's Lynn (3-5-2): Paul Jones; Adam Crowther, Josh Coulson, Tyler Denton; Aaron Jones, Michael Clunan, Theo Widdrington, Ben Stephens (Christian Oxlade-Chamberlain 66), Olly Scott; Gold Omotayo (Ken Charles 55), Josh Barrett (Zain Walker 90). Subs (not used): Kyle Callan-McFadden, Sam Blair (gk).
Boston (3-4-1-2): Sam Long; Zak Mills, Shaun Pearson, Ben Pollock; Keenan Ferguson, Tom Platt, Sam McLintock (Will Atkinson 58), Harry Lewis (Tre Mitford 46); Scott Pollock; Jordan Burrow, Jake Wright (Joe Starbuck 77). Subs (not used): Luke Shiels, Tom Solanke.
Referee: Michael Robertson-Tant.
Attendance: 2,123 (412 visitors).
PIC: Oliver Atkin.