Unlike Sarah Jane's sonic lipstick, scanner watch and robot dog, K-9, Mr Smith was not given to her by the Doctor, but is in fact a Xylok, a crystalline race which crashed to Earth as a meteorite some 60 million years in Earth's past and became trapped underneath the surface of the Earth for thousands of years until the eruption of Krakatoa brought a single, small Xylok to the attention of the geologists of the time. Eventually, Sarah Jane received this apparent crystal from a geologist friend as an aid to her studies in volcanic activity. She consequently found that the crystal could communicate with her laptop, and in doing so it revealed to her that it could help her track alien life and protect the earth if given the resources to do so, leading up to the creation of the supercomputer known as Mr Smith. Mr Smith's main screen displays a crystal-like visualisation when idle, and behind the screen is a large mass of tangled wires and electronics.
Mr Smith is hidden behind the chimney breast in Sarah Jane's attic at 13 Bannerman Road, Ealing, London. It reveals itself on the vocal command "Mister Smith, I need you," although it is capable of self-activation. When it reveals itself as requested a fanfare plays (apparently diegetic, as Sarah Jane comments on it in the Doctor Who episode "The Stolen Earth" but never in The Sarah Jane Adventures). The fanfare is omitted if summoned "Mister Smith, I need you quickly and quietly," or if it independently determines that the fanfare would be inappropriate. Mr Smith can be told to make its voice quieter. It can also request the attention of Sarah Jane and can be heard quite clearly from her living room, a full three floors below. Mr Smith can also be called through mobile phones. In The Gift, Clyde called Mr Smith through K-9.
In "Invasion of the Bane", Sarah Jane is somewhat unwilling to allow either Maria Jackson or the young boy whom they rescued into her attic to see Mr Smith or her other alien technology and momentos. However, after Kelsey Harper sneaks upstairs, Sarah Jane reveals Mr Smith to them when she uses it to hack into the Bubble Shock! factory computers in order to issue a warning to Mrs. Wormwood and the other Bane.
Mr Smith is also able to create replicas of official documents for Sarah Jane's use; for example, it created the necessary documents she needed to adopt Luke and falsified news reports to explain global blackouts. According to the official website Mr Smith has two access levels and Sarah Jane has only basic level clearance, as Mr Smith only allows the species that created it access to level two (although even basic level clearance has a very wide range of capabilities, so Sarah Jane is not hindered by this). It constantly scans all media, police, and other similar sources - even passing space ships - for reports of alien incursions and extraterrestrial information. It also scans for celebrity gossip for its own amusement.
Mr Smith is clearly capable of conducting self-diagnostics and of extrapolating from orders given to generate information likely to be needed. At the end of "Revenge of the Slitheen", he also warns Clyde Langer against trying to investigate its capabilities, and does so in an uncharacteristically authoritarian and somewhat malevolent "voice", suggesting a darker side to its personality as it even gives a slightly menacing chuckle.
In the penultimate episode of the first series, "The Lost Boy", Mr Smith "readopts" its "evil" persona when it reveals to Clyde that it is a Xylok and that it had manipulated the Slitheen into kidnapping Sarah Jane's adopted son Luke. In doing so, it plans to bring the Moon crashing into the Earth by using Luke - subsequent to his impendingly "arranged" escape by Mr Smith - in conjunction with a telekinetic energiser headset, codenamed MITRE (derived from the Greek "mitre"), which is tuned to a human's natural telekinetic power, thereby enabling the user to move things with their mind. The gravitational pull of the Moon would tear the Earth apart, thereby freeing the Xylok race.
Sarah Jane eventually returns to her flat and confronts Mr Smith, who proceeds to monologue in the same malevolent voice as Luke pulses in agony. In retaliation, she unleashes K-9 to battle Mr Smith while she covertly enters an Armageddon code virus acquired by Maria's father. Thanks to K-9's attack distracting Mr Smith, she is successful in her attempts to infect him with the computer virus. As Mr Smith's databanks begin to collapse, it loses its memories of its primary objective, which was originally to release other Xyloks from within the Earth, and Sarah Jane "reaffirms" Mr Smith that its objective is to save the Earth.
Mr Smith shuts down, and the Moon returns to its regular orbit. In the episode's dénouement, Sarah Jane promises that when Mr Smith reboots, its new purpose will be "to safeguard the Earth", meaning it will resume its normal role in the series.
Mr Smith makes an appearance in the two-part Doctor Who episode, "The Stolen Earth", along with Sarah Jane and Luke, helping the two of them to contact other past companions such as Jack Harkness and Martha Jones. When former Prime Minister Harriet Jones asks them to help her send a signal to the Doctor, Mr Smith hacks into every telephone line on Earth, allowing the entire world to phone the Doctor - currently using Martha's old mobile phone - simultaneously, thus breaking through the barrier that Davros and the Daleks had erected around Earth. Mr Smith further assists the team in the finale episode, "Journey's End", with the help of K-9 and Torchwood Three, by creating a lasso of temporal energy around the TARDIS to tow Earth back to its proper location in space.
Since the reboot, his visualisation has changed into a distorted, liquid like, version of his original one and some of his controls have been slightly altered, although in the Doctor Who episodes the visualisation is still the same as in through most of the first series of The Sarah Jane Adventures. It also appears to have gained a sense of humour. While Mr Smith was "bad" at the end of the first series, the crystal shape that rotated, grew, and shrunk on his screen on a black background, turned green in some places and black in others.
Mr Smith appeared to start to self destruct in "Prisoner of the Judoon" when ordered to do so by Sarah Jane while she was under the control of an alien entity, but Luke was able to override the self-destruct by pointing out the conflict with that order and Mr Smith's prime directive to protect humanity, given that the resulting explosion would destroy several city blocks.
It demonstrated displeasure and sarcasm in "The Mad Woman in the Attic" when told that K-9 Mark IV would return to residing with the Smith family. Mr Smith is openly resentful of K-9 and peridically disparages the dog. When Clyde Langer secretly borrowed K-9 in order to cheat on a school exam, he asked Mr Smith not to tell Sarah Jane and Luke; Mr Smith readilly agreed and graciously thanked Clyde for taking the dog.
Mr Smith also has control of two powerful fans which it can power up on command, in "The Gift, Part 2" it severely reduces his power packs. They are located behind the screen of Mr Smith and the screen slides up out the way. Mr Smith states "Every computer has a cooling system."
In one timeline, Mr Smith will "die" [sic] several years before 2059. It is unknown if, in the new timeline, he will be functioning in 2059 when 13 Bannerman Road is the home of Rani Chandra and whether he will still be located in the attic of that house.