The result, coupled with Guiseley's victory over Darlington, relegated the Robins back to the Evo-Stik Northern Premier League.
Tom Clare had handed the Pilgrims a second minute lead at The Jakemans Stadium which was cancelled out by Jamie Reed in the 31st minute, only for Cresswell to score the winner three minutes later.
Clare and Cresswell were the two players coming into the United XI ahead of kick-off, with Ben Davies and Andi Thanoj making way.
CLARE had an immediate impact and swept the ball home from close-range, having initially set Max Wright on his way to force George McMahon into a smart save, with only two minutes on the clock.
Willis, later named the club’s player of the year, soon extended himself to tip Sam Sheridan’s rising drive to safety.
Jay Rollins planted a free header a fraction over the bar for United but the visitors were soon level when REED cut inside to angle the ball across Willis into the far corner a minute past the half-hour mark.
The Pilgrims re-asserted their lead in the 34th minute when Ryan Qualter headed George Smith’s free-kick back across goal and CRESSWELL hooked the ball home from close-range.
The second period presented further opportunities for Cresswell, who headed narrowly off target from one Nicky Wroe corner before seeing a further header nodded off the line by Scott Kay.
McMahon denied Brad Abbott a spectacular third for the Pilgrims while Willis made the save of the game to thwart Reed with 20 minutes to play.
Boston (4-4-2): George Willis; Ben Middleton, Ryan Cresswell, Ryan Qualter, Ashley Jackson; Max Wright (Andre Johnson 87), Brad Abbott, Nicky Wroe (Andi Thanoj 76), George Smith (Ben Davies 82); Tom Clare, Jay Rollins. Subs (not used): Cameron Hawkes, Dylan Parkin (gk).
Ashton (4-3-3): George McMahon; Javid Swaby-Neavin, Toby Lees, Luke Ashworth (Craig Hobson 81), Matt Regan; Connor Dimaio, Scott Kay (Kallum Mantack 68), Sam Sheridan; Liam Goulding, Jamie Reed, Liam Tomsett. Subs (not used): John Roberts, Paul Ennis, Aaron Chalmers.
Referee: Sam Mulhall.
Attendance: 942 (23 visitors).