Former England winger Karen Burke set Leeds on the way to victory with a 20th minute close range effort, her 11th of the season.
Jessica Fishlock equalised from a 38th minute penalty, but a 20-yard scorcher from Ward wrapped up the points for United 15 minutes from time.
Victory took Leeds to the top of the table for the first time, but a seventh defeat in eight matches for Cardiff kept the Bluebirds in the relegation zone.