Eaton was a reserve on his high school basketball team before graduating and working as an auto mechanic.

[4] As a senior at Westminster High School in Orange County, he stood 6 feet 11 inches (2.11 m) and weighed 175 pounds (79 kg) but was uncoordinated, not very muscular, and relegated to a backup role on the basketball team.

[4][5] He returned to Orange County and worked as an auto mechanic, making $20,000 a year, when he was eventually discovered by Tom Lubin while repairing cars in Anaheim in April 1977.

[4][7] After his freshman year at Cypress, Eaton was selected by the Phoenix Suns in the fifth round of the 1979 NBA draft with the 107th pick.

He averaged 14.3 points per game in two seasons at Cypress and led the school to the California junior college title as a sophomore in 1980.

The tallest players in the starting lineup were Darren Daye and Cliff Pruitt at 6 feet 7 inches (2.01 m),[9] but Eaton was too slow for the team's fast-paced offense.

[10] In Eaton's senior year in 1981–82, new coach Larry Farmer vowed to give him a shot to start, but heralded freshman Stuart Gray got the nod instead.

[15][16][17] Because of his lack of playing time at UCLA, few NBA teams had an interest in Eaton after he finished his college career.

[13] However, the Utah Jazz, who finished in last place the prior season,[18] saw him as a potentially dominant defender and selected him as a long-term project in the fourth round of the 1982 NBA draft with the 72nd overall pick.

'"[6][20] Also the team's general manager, Layden discouraged Eaton from playing in Europe and signed him to a five-year contract, with the first season guaranteed at $45,000, for a total $570,000.

[14] He made an immediate impact as a rookie, starting 32 games and replacing Danny Schayes after the cash-strapped Jazz traded the center mid-season.

[6][23] His 3.4 blocks per game ranked third in the NBA, behind Atlanta's Tree Rollins and San Diego's Bill Walton.

[19] During the season, he failed in his attempt to block the hook shot which gave Kareem Abdul-Jabbar his 31,421st point to break the NBA career scoring record held by Chamberlain.

[26][27] Eaton's strong defense helped the Jazz improve from 30–52 in his rookie season to 45–37,[28] winning their first Midwest Division title and making their first playoff appearance.

[32] Although he was not a significant offensive contributor, the Jazz relied heavily on Eaton for his shot-blocking, rebounding, and occasional "tippy toe" dunks.

[19] In addition, he was chosen to play in the 1989 NBA All-Star Game,[4] joining teammates Malone and Stockton on the Western Conference team.

Golden State coach Don Nelson spread out his offense and avoided going inside against Eaton, and they played most of the series with a small lineup in which their tallest players on the court were 6 ft 8 in (2.03 m) Larry Smith or Ben McDonald or even 6 ft 7 in (2.01 m) Rod Higgins.

[35][36] Opponents were increasingly playing with smaller lineups, forcing Eaton to guard a quicker player who would draw him out to the perimeter and seek to drive past him.

Utah coach Jerry Sloan countered by decreasing Eaton's playing time and employing his own small lineup with backup center Mike Brown.

[18] He never averaged more than 10 points per game in a season, which frustrated Utah fans after his scoring tapered off following a career high of 9.7 in 1984–85.

[45] In 2014, Eaton had his jersey retired at Westminster High School and also at Cypress College, along with Swen Nater and head coach Don Johnson.

[52][53][54] He was also a founder and chairman of the Mark Eaton Standing Tall for Youth organization, which provided sports and outdoor activities for at-risk children in Utah.

[58] Around 2016, he began riding a custom French-built road bike for tall cyclists, which came outfitted with 36-inch (910 mm) wheels.