The Pilgrims opened the scoring after six minutes through a trialist.
It was nearly 2-0 moments later when another trialist hit the woodwork before the ball was cleared to safety by Niall Smith.
But it was 2-0 when the visitors were awarded a penalty after Max Pemberton fouled a trialist.
George Green coolly slotted home after sending the keeper the wrong way.
The Tigers’ first opportunity came after thirty minutes when Lynton Karkach whipped a delightful ball towards the back post where Matt Sykes fizzed a volley across goal and wide.
Karkach then sent another ball into the area where Matt Reay’s run ended with the defender forcing a header just over the bar.
David Reay tipped Karl Byrne's fierce effort over on the hour mark as United enjoyed another good spell.
Steven McDonnell pulled a goal back to give the Tigers hope, before Jordan Thewlis rounded off the win with a goal seven minutes from time.