The Pilgrims were no match for last season’s league and cup double winners at Boston Town’s DWB Stadium.
United’s 1-0 half-time deficit soon snowballed and Brentwood ran in three second half goals without reply.
Boston (4-3-3): Travis Portas (Harry Payne 50); Jack McCallum (James Gordon 50), Kofi Ross, Daniel Moulds (Conor Allen 50), Bailey Forth (Elliot Andrew 50); Jack Dykes, Harry Vince, Oli Drummond (George Hobbins 50); Tinashe Chavundura, Harry Limb, Kemal Yenibertiz.