Chelsea were 4-0 up at half-time and the Blues added another two goals in the second half to claim their first silverware of the season at Vicarage Road.
Fran Kirby had a hand in all six goals, scoring twice herself in an outstanding display.
Bristol City went into the match as underdogs and Chelsea were a class above their opponents. It means Chelsea defend the crown they won last year and have now won nine trophies since Emma Hayes took over in 2012.
Chelsea, the WSL leaders and fresh off the back of beating Atletico Madrid in the Champions League in Italy on Wednesday, went ahead after two minutes through Kerr and never looked back.
Kirby and Kerr combined again when Australia star Kerr added her second on 10 minutes and Kirby got in on the act herself with the third goal on 29 minutes.
It was 4-0 after 34 minutes when Kirby cut inside and her shot beat Bristol City goalkeeper Sophie Baggaley.
Kerr completed her hat-trick three minutes into the second half and Kirby set up another one for Guro Reiten to add a sixth on 55 minutes.
Maren Mjelde suffered an nasty-looking injury late on that took some of the shine off a wonderful Chelsea victory, which was the biggest win in a Women’s League Cup final.
Chelsea are targeting the quadruple this season and the Blues will hope this is just the start of what could be an historic season.