Sikma played at Illinois Wesleyan University. He was drafted eighth overall in 1977 by the Seattle SuperSonics, Sikma was named to the NBA All-Rookie Team in 1978. Among his notable career achievements are his seven All-Star Game selections (from 1979 to 1985) and his championship ring won with the 1978–1979 Sonics, where Sikma teamed with talented players such as Dennis Johnson, Gus Williams and an aging Paul Silas. Sikma always averaged double figures in points-per-game throughout his career, and after his stint with the Sonics, he maintained consistent numbers while playing with the Milwaukee Bucks in his final five seasons; for his career he averaged 15.6 points (17,287 in total) and just under 9.8 rebounds per game over 14 seasons and 1107 games.
Sikma was one of the most accurate shooting centers in NBA history. He holds the rare distinction of leading the league in free-throw percentage (92.2%) while playing the center position during the 1987–88 season; he averaged 84.9% in free-throw shooting for his career. Sikma also made over 200 three-pointers during his career with a 32.8% three-point accuracy.
Along with his accurate shooting, Sikma led the league in defensive rebounds in both 1981–82 and 1983–84 while playing for Seattle.
From 2003 to 2007 he worked with the Seattle SuperSonics as an assistant coach.
In June 2007 Sikma was hired by the Houston Rockets as an assistant coach to head coach, Rick Adelman. Among his duties was tutoring center Yao Ming in "big man" playing strategies. On December 6, 2011, he was signed as an assistant coach by the Minnesota Timberwolves, again under Adelman.
Sikma is currently a coaching consultant for the Toronto Raptors, particularly working with Center, Jonas Valančiūnas.
Sikma's son, Luke, played for the University of Portland Pilots, and plays professionally in Spain. As of 2011, his other son, Nate, played for the University of Hartford Hawks men's basketball team.
In 2006, Sikma was voted as one of the 100 Legends of the IHSA Boys Basketball Tournament, a group of former players and coaches in honor of the 100 anniversary of the IHSA boys basketball tournament. On June 27, 2017, Sikma was inducted into the Small College Basketball Hall of Fame as part of their second class . Inducted alongside Sikma were Zelmo Beaty, Walt Frazier, Bob Love, Elmore Smith, Jim Spivey, Rico Swanson, George Tinsley, and Al Tucker.
Sikma's no. 43 jersey was retired by the Seattle SuperSonics, one of only six players to be honored by the team.