The victory moved table-topping City to 49 points after 19 of 22 games with Arsenal, who beat Tottenham Hotspur 5-2 in the late kick-off at The Emirates, rising to second place on 38 points with two games in hand over the leaders.
Arsenal's win saw Manchester United, who also have 38 points, slip to third spot on goal difference, while Chelsea, who have won the last six titles, are fourth with 37 points.
On a day when the weather swung from bright spring sunshine to dark clouds and driving rain, City started off at a frenetic pace at United and did not let up until they had the three points firmly in the bag.
Lauren Hemp smacked a shot off the crossbar in the 16th minute, and Miedema gave City the lead a minute later from the ensuing corner with a looping header that evaded the dive of Phallon Tullis-Joyce in the United goal.
Two minutes later, Miedema scored again as City swept up the pitch with a brilliant passing move, Kerstin Casparij crossing for her unmarked Dutch compatriot to leap into the air and send a downward header bouncing into the net.
Casparij netted the third four minutes after the break, steaming in at the far post to convert after Hemp's cross flew just over the head of City striker Bunny Shaw.
Marc Skinner's United looked a step slow in everything they did, and did not manage an effort on target until late in the second half. It was far too late, though, as City cruised to victory to close in on their first title in 10 years.
"I really believe in my group of players that no matter what team we play, if we play with this confidence, then many teams will struggle against us," City's Swedish coach Andre Jeglertz said.
"Things can still happen in the three games left, and we need to keep performing, but I can't see that pressure being too much because we are so open with it. The players have been very good at handling it."
In a day of derbies in the WSL, Liverpool held on for a 3-2 win at Everton in the early kickoff, while Arsenal's England striker Alessia Russo netted a hat-trick within the first half-hour as her side defeated Spurs before a crowd of 46,123.