Gary Wayne Coleman was born in Zion, Illinois, on February 8, 1968. He was adopted by W. G. Coleman, a fork-lift operator, and Edmonia Sue, a nurse practitioner. Due to focal segmental glomerulosclerosis, a kidney disease, and the corticosteroids and other medications used to treat it, his growth was limited to 4 ft 8 in (142 cm), and his face kept a childlike appearance even into adulthood. He underwent two unsuccessful kidney transplants in 1973 and again in 1984, and required dialysis.

Coleman was an avid railroad fan, and he later worked part-time at Denver-area, Tucson-area, and California hobby stores to be around his hobby. Coleman built and maintained miniature railroads in his homes in several states throughout the 1990s. Currently, at least one of Coleman's model railroads is being preserved in Colorado Springs, Colorado. In a 1993 television interview, Coleman said he had twice attempted suicide by overdosing on pills. Coleman lived in Santaquin, a small town about 50 miles (80 km) south of Salt Lake City, Utah, from 2005 until his death.

In early 2007, Coleman met Shannon Price, 22, on the set of the film Church Ball, where she was working as an extra. Price and Coleman married several months later. On May 1 and 2, 2008, they made a well-publicized appearance on the show Divorce Court  to air their differences in an attempt to save their marriage. However, they divorced in August 2008, and Coleman was granted an ex parte restraining order against Price to prevent her from living in his home when he was hospitalized after their divorce. According to a court petition later filed by Price, she and Coleman continued to live together in a common-law marriage until his death. However, a judge ultimately ruled against Price after hearing testimony that she engaged in love affairs with other men during the time when she claimed to have been with Coleman, and "physically abused Coleman in public, led him around by the hand like a child [and] displayed no physical affection toward him in front of anyone."
In 1998, Coleman was charged with assault against Tracy Fields, a Los Angeles bus driver and fan of Coleman's work on Diff'rent Strokes who had approached him in a California mall and requested his autograph while he was shopping for a bulletproof vest. When Coleman gave her an autograph but refused to personalize it, an argument ensued, and Fields reportedly mocked Coleman's acting career. Coleman then punched Fields in the face in front of witnesses. He was arrested and later defended himself in court, alleging that he felt threatened by Fields. He said: "She wouldn't leave me alone. I was getting scared, and she was getting ugly." Coleman pleaded no contest to one count of assault, received a suspended jail sentence and was ordered to pay Fields' $1,665 hospital bill and to take anger management classes.

In 2007, Coleman was cited for misdemeanor disorderly conduct in Provo, Utah, after a "heated discussion" in public with his wife, Shannon Price. In 2009, Coleman and his ex-wife were involved in a domestic dispute, after which Price was arrested on suspicion of domestic violence, and both parties were cited for disorderly conduct.

In 2008, Coleman was involved in an altercation at a Payson, …

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In 2009, Coleman underwent heart surgery. Although the details of the procedure were never made public, he is known to have developed postoperative pneumonia. In January 2010, Coleman was hospitalized after a seizure in Los Angeles, and in February, he experienced another seizure on the set of The Insider television program. On May 26, 2010, Coleman was admitted to Utah Valley Regional Medical Center in Provo, Utah, in critical condition after falling down the stairs at his Santaquin home and hitting his head, possibly after another seizure, and experiencing an epidural hematoma. According to a hospital spokesman, Coleman was conscious and lucid the next morning, but his condition subsequently worsened. By mid-afternoon on May 27, he was unconscious and on life support. He died at 12:05 pm MDT on May 28, 2010, at age 42.

The weekend after Coleman's death, a scheduled funeral was postponed and later canceled following a dispute regarding the disposition of his estate and remains among Coleman's adoptive parents, former business associate Anna Gray and Price. Coleman's former manager Dion Mial was initially involved but withdrew after Coleman's 1999 will, which had named Mial as executor, was found to have been superseded by a 2005 will replacing Mial with Gray. The earlier will had stipulated that Coleman's wake be "...conducted by those with no financial ties to me and can look each other in the eyes and say they really cared personally for Gary Coleman", but the later version directed "...that there be no funeral service, wake, or other ceremony memorializing my passing."
Questions were raised as to whether Price, who approved discontinuing Coleman's life support, was legally authorized to do so. The controversy was exacerbated by a photograph published on the front page of the tabloid newspaper Globe depicting Price posed next to a comatose, intubated Coleman under the headline, "It Was Murder!" While Coleman's final will named Gray as executor and awarded his entire estate to her, Coleman and Price married in 2007. Although she had divorced Coleman in 2008, Price claimed in a court petition that she had remained his common-law wife, with the two sharing bank accounts and presenting themselves publicly as husband and wife until Coleman's death. Her assertion, if validated by the court, would have made her his lawful heir.

In May 2012, judge James Taylor stated that while Price had indeed lived in Coleman's home after their marriage ended, their relationship at the time of his death failed to meet Utah's standard for a common-law marriage. The hospital later issued a statement confirming that Coleman had completed an advance healthcare directive granting Price permission to make medical decisions on his behalf. An investigation by Santaquin police was closed on October 5, 2010, after the medical examiner ruled Coleman's death accidental and no evidence of wrongdoing could be demonstrated. The disposition of Coleman's ashes remains unknown. Price said that had she been granted disposition, she would have scattered the ashes at the Golden Spike National Historic Site in Utah as a tribute to Coleman's lifelong love of trains.