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A mentalist is an individual who appears to have supernatural powers in divining the truth about an individual as well as many facts about that person's life. A mentalist must be great at decoding, possess observational skills, and have a highly developed ability to observe minute detail. Many people, from criminal profiles to magicians, all use mentalist tactics and a working knowledge of psychology to interpret human behavior. – http://www.wikihow.com/Be-a-Mentalist

The Law of Mentalism from the KYBALION:

This Principle embodies the truth that ‘All is Mind.’  It explains that The All (which is the Substantial Reality underlying all the outward manifestations and appearances which we know under the terms of ‘The Material Universe’; the ‘Phenomena of Life’, ‘Matter’, ‘Energy’, and, in short, all that is apparent to our material senses) is spirit which in itself is unknowable and undefinable, but which may be considered and thought of as an universal, infinite, living mind.

It also explains that all the phenomenal world or universe is simply a Mental Creation of The All, subject to the Laws of Created Things, and that the universe, as a whole, and in its parts or units, has its existence in the Mind of The All, in which Mind we ‘live and move and have our being.’

This Principle, by establishing the Mental Nature of the Universe, easily explains all of the varied mental and psychic phenomena that occupy such a large portion of the public attention, and which, without such explanation, are non-understandable and defy scientific treatment. Excerpt fromhttp://http://attractionlawof.com/applying-the-law-of-mentalism/

History

Much of what modern mentalists perform in their acts can be traced back directly to "tests" of supernatural power that were carried out by mediums, spiritualists and psychics in the 19th century. However, the history of mentalism goes back even further. Accounts of seers and oracles can be found in works by the ancient Greeks and in the Old Testament of the Bible. Among magicians, the mentalism performance generally cited as one of the earliest on record was by diplomat and pioneering sleight-of-hand magician Girolamo Scotto in 1572. The performance of mentalism may utilize these principles along with sleights, feints, misdirection and other skills of street or stage magic.

Mentalists and Magicians …

Mentalists generally do not mix "standard" magic tricks with their mental feats. Doing so associates mentalism too closely with the theatrical trickery employed by stage magicians. Many mentalists claim not to be magicians at all, arguing that it is a different art form altogether. The argument is that mentalism invokes belief and when presented properly, is offered as being "real" be it a claim of psychic ability, or proof that supports other claims such as a photographic memory, being a "human calculator", the power of suggestion, NLP, etc. Mentalism plays on the senses and a spectator's perception of tricks.

Magicians ask the audience to suspend their belief and allow their imagination to play with the various tricks they present. They admit that they are tricksters and entertainers, and know the audience understands it's an illusion and the magician cannot really achieve the impossible feats shown, such as sawing a person in half and putting them back together without injury.

Mentalism techniques have, on occasion, been allegedly used outside the entertainment industry to influence the actions of prominent people for personal and/or political gain. Famous examples of accused practitioners include:
• Erik Jan Hanussen, alleged to have influenced Adolf Hitler
• Grigori Rasputin, alleged to have influenced Tsarina Alexandra
• Wolf Messing, alleged to have influenced Joseph Stalin
• Count Alessandro di Cagliostro, accused of influencing members of the French aristocracy in the Affair of the Diamond Necklace
"The Amazing Kreskin" has audience members hide his cheques before the show; if Kreskin cannot find the cheque at the end of his performance, he does not get paid. *

The ‘How’ of it …

Mentalism is a much newer type of magic than most of the other forms of magic tricks out there but it’s also one of the most impressive. These days just about everyone has seen a rabbit being pulled out of a hat or someone successfully guessing the card that they picked out of a deck but something that most people have never seen is mentalism magic and when they first do it completely blows them away.
This is because mentalism gives the illusion that the mentalist can really read someone’s mind and it’s almost scary when someone first witnesses an illusion of this type because it seems downright impossible and as if someone is actually inside of their head.

But really mentalism is just a clever science that revolves around understanding human behavior and predicting different outcomes to mimic the act of reading somebodies mind.

Mentalism became extremely popular more recently thanks to mental magicians like Derren Brown who have performed incredible mentalism tricks on television but it’s still left most people wondering “just how on Earth did he do that?”
Well now we can reveal to you the 3 biggest mentalism secrets out there to give you some insight into how these tricks really work.

Secret #1 – Watch Their Lips
A mentalist will often tell someone in their audience to think of a word or item and to repeat it over and over again. Then as the mentalists rambles on about something or other and the person is saying that word in their head repeatedly they’ll actually very slightly move their lips in the way that the word that they are thinking of is said without actually saying it and all without ever realizing it.
If the mentalist is observant they can pick up on this and guess the word or item that the person is thinking of. The best way is to focus on picking up the first and last letters of the word after which is becomes relatively easy to fill in the middle and successfully guess the word.

Secret #2 – Misdirection Is Everything
By far one of the best ways to convince somebody that you are reading their thoughts and are a true mentalist is to plant a thought into their mind so that you know full well what it is and this can be done through misdirection. Misdirecting is when you through words and body language get the subject to subconsciously focus on something so that when you ask them to think of something that is what they think of.
For example if you subtly work a work in several times into your routine such as ‘watch’ and then quickly glance at your watch as you are doing the routine and do this in the right way then if you go to ask that person to “think of an accessory” they will almost always think of a watch.
Furthermore that person will think that they thought of it and that you had nothing to do with it. Basically this works because people are much more suggestible than they would realize or often like to admit!

Secret #3 – Fish For Information
A great way to extract information from a subject is by fishing for. Fishing for information is done by saying broad and general statements that seem to the subject as if you have somehow read their mind when really it could apply to almost anything or anyone.
For example many mentalists will say something along the lines of “I’m seeing the number 19 really strongly… does this mean something to someone?” to their audience and sure enough somebody will have a connection with that number in some way and step forward after which you can fish for more information and more information until you have seemingly read their mind!

Other sources:
*https://en.wikipedia.org/wiki/Mentalism
** https://www.mentalismzone.com/3-biggest-mentalism-secrets-exposed/