Mamoru went to Harvard in the United States of America to study abroad. However, before he could get there, Galaxia attacked his plane mid-flight and removed his star seed, killing him. Whether the rest of the flight was affected isn't shown, and Usagi is not shown to have any awareness of what happened to Mamoru throughout the season.
In the manga's final arc, "Stars," something similar happens in that Galaxia removes Mamoru's Sailor Crystal, but before he boards the plane. Usagi witnesses the entire thing, and faints from shock. She doesn't quite believe what she has seen, even when it is revealed that the person sending her mysterious cards is not Mamoru, as she expected, but Seiya.
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No. As is already apparent from the arcs depicting Neo-Tokyo, she marries Darien (Mamoru). Thus Reeni (Chibiusa).
*WARNING SPOILER!* Darien/Mamoru is actually dead. *sniff* Galaxia stole his Star Seed when he was on the airplane on his way to America. Sailor Moon finds out in episode 197 or so. He finally returns at the end of episode 200.
Mamoru (Darien) leaves in the heartbreaking episode 173 season 5. :)
Mamoru is in his early 20s in the anime.
In sailor moon Endymion is the prince of earth but instead of Endymion he know as Darren or Mamoru.