- Before he was Professor X, he was Charles. Before he was Magneto, he was Erik. Before they were enemies, they were allies
- X-Men (First appearance chronologically) (Origin revealed)
- Magneto (Erik Lehnsherr) (First appearance chronologically) (Joins and leaves team)
- Professor X (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) (First appearance) (Joins Team)
- Banshee (Sean Cassidy) (First appearance) (Joins Team)
- Darwin (Armando Muñoz) (Only appearance; dies)
- Hellfire Club (First appearance)
- Brotherhood of Mutants (First appearance chronologically)
- Edie Lehnsherr (Death)
- Colonel Bob Hendry (Only appearance; dies)
- CIA Director John A. McCone (First appearance)
- William Stryker, Sr.
- William Stryker, Jr. (Mentioned)
- James Howlett (Cameo)
- John F. Kennedy (Appears on a Computer Screen, TV or Hologram Only)
- Various Soviet soldiers and military officers
- Various American Navy soldiers and officers
- United Kingdom
- Soviet Union
- United States of America
- Magneto's Helmet (First appearance chronologically)
- Cerebro (First appearance chronologically) (Destruction)
- X-Jet (First appearance chronologically) (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
- Michael Fassbender as Magneto / Erik Lehnsherr
- January Jones as Emma Frost
- Caleb Landry Jones as Banshee / Sean Cassidy
- Lucas Till as Havok / Alex Summers
- Nicholas Hoult as Beast / Hank McCoy
- Jennifer Lawrence as Mystique / Raven Darkholme
- Kevin Bacon as Sebastian Shaw
- Edi Gathegi as Darwin / Armando Muñoz
- Jason Flemyng as Azazel
- Rose Byrne as Moira MacTaggert
- Álex González as Riptide / Janos Quested
- Oliver Platt as "The Man in Black"
- Zoë Kravitz as Angel / Angel Salvadore
- Hugh Jackman as Logan (Cameo)
- Éva Magyar as Magneto's mother.
- Bill Millner as the young Magneto
- Beth Goddard as Sharon Xavier
- Laurence Belcher as the young Charles Xavier
- Morgan Lily as the young Mystique
- Rebecca Romijn as the adult Mystique (Cameo)
- Don Creech as William Stryker, Sr.
- Glenn Morshower as Col. Bob Hendry
- Matt Craven as CIA Director John A. McCone
- James Remar as U.S. General
- Rade Šerbedžija as Soviet General
- Michael Ironside as U.S. Navy Captain
- Ray Wise as U.S. Secretary of State
- 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. 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 tuned in to 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.
- In X-Men: The Last Stand, Erik is present when Charles recruits Jean, ten years before the main time-line where Charles is also still able to walk. He was also still able to walk in X-Men Origins: Wolverine, when he recruited Scott and other mutants fifteen years before the main time-line and again, in X-Men: Days of Future Past (film), it is shown that with the same injections used by Hank, Charles can also control his mutation and walk again, what doesn't explain how did he use them in X-Men: First Class.
- 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.
- Hugh Jackman makes an uncredited cameo as Wolverine. He's the cigar smoking man Charles and Erik approach in the bar who tells them to "Go fuck [them]selves" after they introduce themselves. This is referenced in X-Men: Days of Future Past.
- 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.
- Characters from X-Men: First Class (film)
- Other things related to X-Men: First Class (film)
- Film Gallery: X-Men: First Class (film)
- Images from the film