A low-key first half was enlivened in the 39th minute by a superb finish from Eoin Lanigan as he found the net with a crisp first-time volley.
The game was put beyond Athlone them when centre back Des Hope drove into his own net when attempting to clear a cross from Monaghan substitute Andrew Haran (77).
Haran, making his home debut, wrapped things up for United in the 83rd minute, running on to David Scully’s through ball and beating Ciaran Kelly in the Athlone goal with a deft finish.
Monaghan United: Kennedy; McMahon, Mooney, Gartland; Whelan, Reilly (McCrossan 82), Tierney (Williamson 85), Brennan, Clancy; Lanigan (Haran 69), Scully.
Athlone Town: Kelly; Rossiter, Keady, Hope, Reilly (Hamm 77); Maloney, Jinks, Russo, McCartney (Lavine 36); Mernagh (McGee 65), Coffey.
Referee: H Whoriskey.