Graham Bayne scored twice as Arbroath leapfrogged 10-man Forfar to move up to fourth in the Scottish Second Division table.
Lee Sibanda put the hosts ahead after eight minutes, converting Bayne's clever headed knockdown.
Bayne opened his own account with a close-range strike just nine minutes later as the hosts pressed home their first-half dominance.
Striker Steven Doris laid on the third 10 minutes after the interval, Bayne again profiting as he steered the ball in from six yards out.
The Loons gave themselves a glimmer of hope when Colin Hamilton fouled Willie Robertson in the box and Iain Campbell converted the penalty.
However Robertson was given his marching orders with 11 minutes left after a poor challenge on Hamilton.
Arbroath: Bullock, Baxter (Scott Robertson 61), Malcolm, Keddie, Hamilton, Sibanda, Chisholm (Currie 70), Kerr, Sheerin, Doris, Bayne (Holmes 87). Subs Not Used: Morrison, Rennie.
Booked: Sibanda, Sheerin, Bayne, Bullock.
Goals: Sibanda 8, Bayne 17, 56.
Forfar: Hill, Robertson, McCulloch, Faeroe, Iain Campbell, Hilson, Fotheringham (Malin 74), Brown, Denholm (Tulloch 62), Templeman (Ross Campbell 68), Swankie. Subs Not Used: Hamilton, King.
Sent Off: Robertson (79).
Goals: Iain Campbell 72 pen.
Ref: David Somers