Mesmero was originally a small time crook who posed as a hypnotist at high society parties. He would use his mutant powers to 'mesmerize' guests into giving him their possessions and then making them forget about them. Mesmero drew the attention of Magneto, who wanted him to hypnotize Lorna Dane into believing that she was Magneto's daughter.
Mesmero succeeded in performing this task, however, a Sentinel attack revealed that the Magneto who had approached Mesmero was actually a robot. Mesmero then attacked the X-Men, the real Magneto appeared. The Master of Magnetism rendered Mesmero unconscious and stranded him in South America.
Mesmero has encountered the X-Men on a few other occasions as well as teams such as Excalibur, Fenris, X-Factor, the Dark Riders, and Alpha Flight.
When Mesmero joined the latest incarnation of the Weapon X program, the Director offered to increase Mesmero's hypnotic powers. Mesmero agreed and gained the ability to control entire crowds with a mere glance, instead of a mere handful of people.
To keep the public unaware of the "Neverland" mutant concentration camp, Mesmero posed as a government liaison. With this identity, he informed news reporters that "Neverland" doesn't exist nor is the government involved in abducting mutants.
While he was a member of Weapon X, Mesmero visited his dying mother. He had doctors help her as much as they could and used his powers to mesmerise his mother into believing she was healthy as ever. His mother knew it was an illusion, though, and she told him that she knew shortly before she died. Subsequently, Mesmero began to lose confidence in himself which caused him to lose his powers. Mesmero was transported to Neverland upon the Director's discovery of his power loss.
Brent Jackson, planning on raising a coup on Weapon X, rescued Mesmero. Mesmero spent several months in seclusion as Jackson helped him get his confidence back, and with it his powers.
Spoiler warning: Plot and/or ending details follow.
Ironically, Mesmero eventually lost his powers for good (along with most of the world's mutants) when the Scarlet Witch altered reality at the climax of the House of M event. Devastated and ruined, the once-great supervillain was forced into poverty.
Mesmero finally achieved a personal triumph by forming a relationship with a woman who had saved his life, promising to somehow help her in turn. She put her trust in him freely and unconditionally, something he had never managed before without the aid of his powers. At this, Mesmero resolved to put his past behind him and begin living as Vincent.