The result ensured City maintained a 100% record in the FA Women's Super League and with 18 points from the opening six games, the Blues have a two point cushion over second-placed Chelsea.
Picked out by Nikita Parris' centre, the Danish forward powered home a header from eight yards and then turned provider for Emslie, who galloped clear and calmly slotted a second.
The Royals were undeterred, however, and scored twice in seven minutes to level the score.
Remi Allen capitalised on Abbie McManus' hesitancy to score from close range before Brooke Chaplen arrowed an effort into the bottom right corner from 20 yards.
Christiansen ensured City went in ahead at the break, after good work from Parris.
The England midfielder arrived late in the penalty area and her deflected effort completed the scoring in an action-packed first half.
Scott confirmed the three points with two goals in two minutes, arcing a header into the far corner for her first, before she arrived at the back post tap home Emslie's low cross.