Son of Pauline and Jackson Smith, Rose Tyler's on-off boyfriend Mickey initially found himself drawn in to the Doctor's world when he was just a young boy, at a point when he hadn't even knowingly met him yet. Attacked by the Reapers in a playground in 1987 Mickey sought refuge in a nearby church (Father's Day) He was instinctively drawn to one of the women there for protection - a woman he could always trust, and would never, ever forget Rose Tyler...
As a child he was abandoned by his mother, with his father disappearing to Spain not long after, leaing Mickey in the care of his blind Grandmother, Rita-Anne. Following her death some years later, Mickey left school to become a car mechanic and moved into a flat on the Powell Estate. When Mickey first encountered the Doctor, he was captured and replicated by the Nestene Conciousness but was later rescued although abandoned on Earth when Rose chose to leave him to explore the universe with the Doctor (Rose).
Mickey didn't date anyone else in Rose's absence, and waited an entire year for her to return. During this time, he was accused by Jackie Tyler of being responsible for her daughter's disappearance, and was taken in for questioning by the police five times. He started researching the Doctor, uncovering details of his past visits to Earth, and his previous involvement with UNIT. When the Doctor and Rose did eventually return to Earth in 2006, Mickey was quickly snubbed by them both, with the Doctor nicknaming him Ricky and Mickey The Idiot in an attempt to irritate him.
Although initially hesitant to support the Doctor's cause, Mickey was able to use his advanced computer skills to hack in to the Royal Navy and UNIT computer sytems during the Slitheen invasion on Earth and hijacked a missile, using it to destroy the Slitheen inside Downing Street ( Aliens Of London, World War Three). Rose asked him to take her passport to Cardiff, where the TARDIS was refuelling, and he helped her, the Doctor and Captain Jack Harkness defeat the surviving Slitheen, Blon Fel Fotch (Boom Town).
Mickey was delighted when the Doctor sent Rose home from the Dalek invasion of 200,100, but eventually accepted that her place was with the Doctor and helped her reactivate the TARDIS and return to the Game Station (Parting Of The Ways).
Missing Rose terribly, Mickey took the opportunity instead to keep Jackie company, visiting her every Sunday for dinner, and helping out with odd jobs around the flat. He was with Rose when the TARDIS was transported aboard the Sycorax ship (The Christmas Invasion) Now accustomed to saying goodbye to Rose (New Earth), Mickey continued to work as a mechanic, and considered himself to be the Doctor and Rose's 'Man In Havana', carrying out surveillance and technical support from afar. In reality, he gradually realised, he was their 'tin dog'.
Mickey eventually joined Rose and the Doctor aboard the TARDIS following their encounter with the Krillitanes (School Reunion), before travelling first to a 51st century spaceship, the SS Madame de Pompadour (The Girl In The Fireplace), before crash-landing on a parallel version of Earth in the present day.
Mickey later chose to stay behind in 'Pete's World' in order to look after 'his' grandmother following the death of her real grandson, Mickey's alter-ego, Ricky Smith (The Rise Of The Cybermen, The Age Of Steel).
He then assisted Pete Tyler and Jake Simmonds in their mission to liberate the world from the remaining Cybermen scattered across the globe, but this attempt was frustrated when the Cybermen used Torchwood technology to travel across the Void in to Rose Tyler's universe. Mickey followed them, masquerading as Samuel in the Torchwood Institute, and was reunited with the Doctor and Rose in the events leading up to the Battle Of Canary Wharf. He was then safely transported back to 'Pete's World' before the Doctor sealed the breach between the universes, and he lived with Pete Tyler and his adopted family Jackie and Rose Tyler. (Army Of Ghosts / Doomsday).
He, along with Rose and Jackie used a Dimension Jump to get back to their own universe to help the Doctor stop the Daleks. Transported to the Dalek Crucible with Jackie and Sarah Jane Smith, he narrowly avoided being killed in a testing of the Reality Bomb. His grandmother on 'Pete's World' had passed away, so he remained on his home Earth after the Daleks defeat, leaving the TARDIS alongside Captain Jack and Martha Jones. (The Stolen Earth / Journey's End).
Some time later Mickey married Martha and together they worked tracking down dangerous aliens on Earth, including a Sontaran Commander, Jask. Jask was close to killing them both, when the Doctor and arrived and stopped him. Mickey and Martha realised this was the very last time they would see the Tenth Doctor - he had come to say goodbye. (The End Of Time - Part II)