After bulldozing their way to the Women's Super League title on the back of Bunny Shaw's 21 goals, the Jamaican international was instrumental again and, after rumours that she might depart, she celebrated signing a new contract with City by opening the scoring in the 38th minute.
Brighton, who beat City in the league run-in but could not prevent them from being crowned champions, started well in their first trip to the final but the wheels came off when Alex Greenwood added a second goal deep into first-half stoppage time to send City in 2-0 up at the break.
Aoba Fujino netted the third in the 66th minute, ghosting past two Brighton defenders and firing home with her left foot, and Dutch striker Vivianne Miedema headed the fourth three minutes before full time to complete the rout.