Reiten gives Chelsea edge over defending champions Lyon
The Norwegian curled home after 28 minutes in France to hand the Blues a 1-0 lead ahead of the reverse fixture in London.
Lyon were guilty of missing a number of opportunities in the first half, although Lauren James could have added a second for Chelsea when she hit the post.
Delphine Cascarino similarly struck the woodwork for Lyon after half-time, with the eight-time champions now needing a turnaround in the second leg if their title defence is to continue.