Mary Jane was first referenced, and has been a part of the Spider-Man franchise, since Amazing Spider-Man Vol. 1 #15. Peter (and the readers) didn't officially meet her until ASM (Vol. 1) #42, but a character doesn't have to actually appear to leave an impact (or I guess they could appear with their face obscured). Saying she "didn't appear until much later" is incorrect. It was this anticipation to Peter meeting her that left a huge impact on readers.
To put this in perspective, the Green Goblin first showed up in Amazing Spider-Man (Vo. 1) #14. Norman Osborn made his unnamed debut in ASM (Vol. 1) #23. He was named in ASM (Vol. 1 #37). And he was unmasked as the Green Goblin in ASM (Vol. 1) #39.
The first mention of MJ to Peter meeting her took longer than it took for the Green Goblin to be unmasked.