The Pilgrims trailed Loughborough Dynamo at The Jakemans Stadium courtesy of Bill Sneath's strike two minutes before the break.
United soon overturned the deficit though with Smith striking 16 seconds after the restart and Forth and Lawrence adding headed goals on 51 and 58 minutes respectively.
Boston (4-3-3): Harry Payne; James Gordon, Jack Withers, Bailey Forth, Callum Rodger; Jordan Tate, Harry Vince, Peter Evison; Ben Lawrence (Jenson Ball 82), George Smith (George Hobbins 73), Jack Brown (Connor Stocks 77).