Hemp's goal early in the first-half at Wembley sealed England's third successive victory, moving them above holders Spain to the top of group A3.
It was the first time the teams had faced each other since the Lionesses' penalty shoot-out victory in last year's European Championship final.
Only the winners from each of the League A groups will earn direct passage to next year's tournament in Brazil.
Keira Walsh, who wore the captain's armband in place of the unavailable Leah Williamson, celebrated her 100th England cap, and Euro 2022-winning goalkeeper Mary Earps was honoured in an international retirement ceremony.
England took just three minutes to go in front as Alessia Russo scooped the ball into the path of Hemp, who hooked it past Cata Coll before Alexia Putellas could clear off the line.
Spain threatened an equaliser when Irene Paredes nodded just over.
Hemp was inches away from doubling England's lead in the 19th minute, clipping the post after she was set up stylishly by Lucy Bronze's backheel pass.
Hannah Hampton preserved England's lead at the halfway point when she denied Patricia Guijarro at the near post.
England escaped early in the second half when Olga Carmona's effort took a deflection and cannoned off the crossbar.
Salma Paralluelo forced a good save from Hampton as Spain poured forward, and the inspired Hampton had the last word as she turned Edna Imade's header past her post in the final minutes.