He was known for his trademark reverse pivot and step back behind-the-head jumper, coined as the "Sikma move",[1] along with his distinctive blond hair during his playing days, which he wore in a pageboy early in his career and in a curly style later on.

[6] After being heavily recruited by many Division I schools, Sikma was swayed by his close relationship with Illinois Wesleyan coach Dennie Bridges and being able to immediately play as a freshman.

[4][7][8] At Illinois Wesleyan, Sikma was a three-time National Association of Intercollegiate Athletics (NAIA) All-American and averaged 27.0 points and 15.4 rebounds as a senior.

Sikma was chosen as the College Conference of Illinois and Wisconsin (CCIW) Most Outstanding Player for three straight years.

Due to the relatively small size of his alma mater, Sikma was considered an unknown quantity compared to the "established, well‐publicized stars" taken before him.

[14][15] In the 1979–80 season as the reigning NBA champions, the Sonics would win 56 games but would fail to defend their title, falling to the Los Angeles Lakers 4–1 in the Western Conference Finals.

Sonics General Manager Les Habegger remarked, "After careful consideration, we concluded that we could look far and wide and never obtain another center of his caliber.

In nine seasons and 715 games with Seattle, Sikma averaged a double-double of 16.8 points, 10.8 rebounds, along with 3.3 assists, 1.1 steals and 1.0 blocks, shooting 47% from the field and 83% from the line.

[21] Later in response, Sports Illustrated would report: Sikma was once a member of that elite caste of NBA players considered "untouchables," stars so closely identified with the city they play in that trading them would be unthinkable.

Four years ago someone asked the Sonics' then-general manager, Zollie Volchok, if he would consider trading Sikma for Moses Malone.

[22]Milwaukee Coach Don Nelson felt Sikma was the missing piece of a team that had frequently dominated the regular season before being exposed at the center position in the playoffs.

[23] In the initial playoff series, Sikma scored 11 points in the fourth quarter to get past Charles Barkley and the Philadelphia 76ers.

[34][35] In five seasons and 392 games with Milwaukee, Sikma averaged 13.4 points, 7.9 rebounds and 2.9 assists, shooting 45% from the floor and 88% from the line.

[42] Towards the end of his career where his lack of athleticism prevented Sikma from using his eponymous move and no other big men appeared to be able to replicate it, he stated that coaches were inquiring him about the technique, even though he believed it would be defunct upon his retirement.

[43] Sikma's overall strength and height, combined with his shooting ability and rebounding become a more common prototype long after his retirement, with protegees like Yao Ming and Kevin Love becoming quality players in the NBA.

[44] On a January 2019 NBA TV segment, former players Chris Webber, Isiah Thomas, and Kevin McHale discussed his unexpected influence on modern-day basketball play with a segment entitled, "Jack Sikma's moves still being used today", discussing common moves taller players have since adopted to utilize their shooting and passing abilities.

[56] American speed skater Bonnie Blair cited Sikma as an early sponsor before she won her Olympic medals.