City got off to the best possible start at the Academy Stadium, finding themselves two goals to the good inside four minutes.
Lauren Hemp powerfully headed home the opener the at the back post inside two minutes, before turning provider for Ellen White moments later, cutting the ball back for the England forward to smash home a second.
While Gareth Taylor's side continued to dominate possession, Fiorentina began to gradually grow into the game, with the second half much more evenly matched.
However, substitute Sam Mewis provided further breathing space for City ahead of next Thursday's second leg clash in Florence when she headed home a third with stoppage time approaching.
Relive the best of the action by watching the video above.