The original quartet of Kathleen Dawson, Adam Peaty, James Guy and Freya Anderson were more than two seconds clear of the rest of the field in the heats, setting a new European record, and Team GB, with the addition of Hopkin for Anderson, eased to victory on Saturday morning.
Here are some pictures from an historic golden moment ...
Anna Hopkin of Team Great Britain reacts after winning the gold medal
All smiles - Anna Hopkin of Team Great Britain reacts after winning the gold medal
Anna Hopkin (right) of Team Great Britain celebrates winning the gold medal in the Mixed 4 x 100m Medley Relay Final at Tokyo Aquatics Centre
Gold medalist Kathleen Dawson of Team Great Britain (right) places the medal of Anna Hopkin of Team Great Britain around her neck during the medal ceremony