First Appearance: X-Men #1 (September 1963).
Appearances: Too many others to list here.
Years Active: 1950s? to the present.
Although Charles Xavier is better known as Professor Xavier of the X-Men, he also led an adventurous life before he founded the X-Men and before the events of Fantastic Four #1.
Xavier's telepathy manifested itself when he was a teenager, before he served in the Korean War; there's never been any evidence, though, that Xavier used his powers during the War. Among the people Xavier served with was William Lonestar, later the father of Danielle (Psyche/Mirage/Moonstar) Moonstar. Sometime during the mid-1950s, following his discharge from the American Army (he was badly wounded in combat), Xavier fought the evil mutant Amahl Farouk, later the Shadow King, and encountered the young Ororo Munroe. (see the Note below) In the late 1950s (as I reckon Xavier's history, anyhow) Xavier and Magneto encountered and defeated Baron Strucker in Israel. After he left Israel Xavier travelled to India and then to Tibet, where he fought and defeated the alien Lucifer, but only at the cost of losing his mobility. At some undefined point in the past Xavier encountered Grace and Cuckoo, two members of the Clan Destine, and helped them contain a dimensional breach by the demon Synraith; they thanked him by removing his memories of the event.
Note: About the time discrepancies here...given the difficulties with time in the Marvel Universe, it's very risky to say that any currently active character was active decades ago. But Xavier's time in the Mediterranean is specifically linked to his stay on Kirinos, which is in turn linked to his recovery from the Korean War. Unless you want to say that Xavier's recovery took decades, or retcon this period of his life away, you have to put his encounters with both the Shadow King and with Storm in the mid-1950s. It's possible, of course, to say today that Xavier was injured in the Vietnam War, and shift all of these events forward 20 years, but the inevitable time rollback will eventually skotch that, too.