- X-Men (First appearance chronologically) (Origin revealed)
- Magneto (Erik Lehnsherr) (First appearance chronologically) (Main story and flashback) (Joins and leaves team)
- Professor Charles Xavier (First appearance chronologically) (Joins team)
- Mystique (Raven Darkholme) (First appearance chronologically) (Joins and leaves team) (Impersonates Sharon Xavier, William Stryker, Sr., Banshee, Sebastian Shaw)
- Beast (Dr. Hank McCoy) (First appearance chronologically) (Joins team)
- Havok (Alex Summers) (Only appearance) (Joins team)
- Banshee (Sean Cassidy) (Only appearance) (Joins team)
- Darwin (Armando Muñoz) (Only appearance; dies)
- Hellfire Club (Only appearance) (Disbands)
- Brotherhood of Mutants (First appearance chronologically)
- Edie Lehnsherr (Death)
- Jakob Lehnsherr
- Sharon Xavier (First appareance; photo)
- Norman (Only appearance)
- Amy (Only appearance)
- Colonel Bob Hendry (Only appearance; dies)
- William Stryker, Jr. (Mentioned)
- John F. Kennedy (Appears on a computer screen, TV, or hologram only) (Topical reference)
- James Howlett (Cameo)
- Various Soviet soldiers and military officers
- Various American Navy soldiers and officers
Races and Species:
- Switzerland (First appearance)
- United Kingdom (First appearance)
- Soviet Union (First appearance)
- Turkey, Asia (Mentioned)
- North America
- United States of America
- New York
- Harvard University, Cambridge, Massachusetts (Mentioned)
- Las Vegas
- Hellfire Club (First appearance)
- Las Vegas
- Virginia (First appearance)
- Washington, D.C.
- Florida (First appearance)
- Miami (First appearance)
- Cuba (First appearance)
- Bay of Pigs (First appearance)
- United States of America
- South America (First appearance)
- Argentina (First appearance)
- Villa Gesell (First appearance)
- Argentina (First appearance)
- Magneto's Helmet (First appearance chronologically)
- Cerebro (First appearance chronologically) (Destruction)
- Banshee's Wings (First appearance)
- X-Uniforms (First appearance chronologically)
- X-Jet (First appearance chronologically) (Destruction)
- Caspartina (Only appearance; destruction)
Erik Lehnsherr, the young Magneto, is forced to live in a death camp during the Holocaust. Erik pulls apart a metal gate when the Nazi soldiers separate him from his parents. Seeing this, the Nazi collaborator and secret mutant Sebastian Shaw becomes fascinated by the boy's potential and attempts to get him to move a German coin. When this fails, he has his mother brought in, threatening to kill her if he does not succeed in moving the coin. When this does not work either, he shoots the woman, causing Erik to tear apart the lab in horror.
Professor Charles Xavier and his associate, Raven Darkholme, are contacted by agents of the CIA to help investigate the Hellfire Club, a clandestine organization of mutants with suspected criminal operations. The association with the CIA soon brings Xavier into contact with other mutants, including Erik Lensherr, Havok, Darwin, Beast and Banshee.
Xavier's mutant group then comes into conflict with the Hellfire Club, which leads to the murder of Darwin at the hand of the Hellfire Club's leader. Xavier resolves to stop the Hellfire Club and begins training his young group in preparation for conflicts to come. During subsequent confrontations with the Hellfire Club, Erik and Charles' views regarding the path of the emerging mutant race diverges, resulting in the end of their friendship and the fracturing of their emerging team. Erik, now going by the name "Magneto", leaves the team to form the Brotherhood of Mutants, with Darkholme following him, while Xavier continues to shape the others into a team with a higher purpose: the X-Men.
- James McAvoy as Professor Charles Xavier
- Laurence Belcher portrays a 12-year old Charles Xavier
- Michael Fassbender as Magneto / Erik Lehnsherr
- Bill Millner portrays a young Erik Lehnsherr
- Rose Byrne as Moira MacTaggert
- Jennifer Lawrence as Mystique / Raven Darkholme
- Morgan Lily portrays a 10-year old Raven Darkholme
- January Jones as Emma Frost
- Nicholas Hoult as Beast / Hank McCoy
- Oliver Platt as "The Man in Black"
- Jason Flemyng as Azazel
- Lucas Till as Havok / Alex Summers
- Edi Gathegi as Darwin / Armando Muñoz
- Kevin Bacon as Sebastian Shaw
- Caleb Landry Jones as Banshee / Sean Cassidy
- Zoë Kravitz as Angel / Angel Salvadore
- Matt Craven as CIA Director John A. McCone
- Álex González as Riptide / Janos Quested
- Rade Šerbedžija as Soviet General
- Glenn Morshower as Col. Bob Hendry
Additionally, Beth Goddard and Éva Magyar appear as Sharon Xavier and Edie Lehnsherr, respectively. Georg Nikoloff portays Erik's father. Annabelle Wallis appears as Amy, a woman with heterochromia. Don Creech portrays William Stryker, Sr. James Remar, Michael Ironside, and Ray Wise appear as a U.S. General, a U.S. Navy Captain, and the U.S. Secretary of State, respectively. Hugh Jackman and Rebecca Romijn make uncredited cameos as James "Logan" Howlett and the adult Mystique, respectively.
- X-Men: First Class frequently contradicts information established in previous X-Men films.
- In X-Men it is stated that Erik Lehnsherr helped Charles Xavier build Cerebro, whereas in X-Men: First Class the designer is Henry McCoy. However, this version of Cerebro is destroyed by Riptide, so it is plausible that Erik helped rebuild a newer version.
- Also in X-Men, Professor X tells Wolverine that Cyclops, Storm, and Jean Grey were his first students, when none of them appeared in First Class. Though they may have been among his first students after reopening the school. Xavier also tells Wolverine that he met Erik Lehnsherr when he was seventeen, but in First Class they are both adults.
- In X2, a television in a bar is turned into a conversation about mutant rights between Dr. Hank McCoy (who still has normal human skin, not blue fur) and a "Mr. Shaw" (long speculated to be Sebastian Shaw, but his first name is not revealed). However, in X-Men: First Class, McCoy has further mutated into his furry blue form, while Shaw has been killed. But in X-Men: Days of Future Past, it is shown that Hank can control his mutation with injections. It's possible "Mr. Shaw" is a relative of Sebastian Shaw (he could be Shinobi Shaw).
- The Emma Frost from X-Men: First Class and the character of Emma from X-Men Origins: Wolverine do not appear to be the same character. The First Class Emma is portrayed as being an adult in the 1960s, with the classic diamond skin and telepathic powers of her comic book counterpart. The Origins Emma is portrayed as a teenager in the 1980s, and while sharing the diamond skin of the First Class Emma, she is not shown to have any on-screen telepathic powers.
- In the comics, Alex Summers is the younger brother of Scott Summers. In X-Men: First Class however, Alex is apparently much older than Scott, and no familial relationship between the two is mentioned. However, in X-Men: Apocalypse, they were indeed revealed to be brothers.
- Sheldon Turner earned a story credit due to the film covering the same setting as his discarded X-Men Origins: Magneto script. Bryan Singer has stated he never read Turner's screenplay. In turn, the contributions by Josh Schwartz and Jamie Moss went uncredited.
- Although it is made clear in the credits and from behind-the-scenes information that the character played by actor Álex González is Riptide, the film makes no mention of this. The character is never referred to by name and has no spoken dialogue.
- X-Men: First Class is the only X-Men film in the original timeline that is unaffected by the changes to the timeline in X-Men: Days of Future Past.