Haywood attended Trinidad State Junior College in Trinidad, Colorado, during the 1967-68 college season, where he averaged 28.2 points and 22.1 rebounds per game. Due to his exceptional performance and talent, Haywood made the USA Olympic Basketball team in 1968. Haywood was the leading scorer on the USA's gold medal winning basketball team during the 1968 Olympics at 16.1 points per game, and he set a USA field goal percentage record of .719.
Haywood transferred to the University of Detroit in the fall of that year, and led the NCAA in rebounding with a 21.5 average per game while scoring 32.1 points per game during the 1968-69 season. He decided to turn pro after his sophomore year, but National Basketball Association (NBA) rules, which then required a player to wait until his class graduated, prohibited him from entering the league. As a result, he joined the Denver Rockets of the American Basketball Association (ABA).
In his rookie season, Haywood led the ABA in scoring at 30.0 points per game and rebounding at 19.5 rebounds per game while leading the Rockets to the ABA's Western Division Title. In the playoffs, Denver defeated the Washington Capitols in 7 games in the Western Division Semifinals before falling to the Los Angeles Stars in the division finals, 4 games to 1. He was named both the ABA Rookie of the Year and ABA MVP during the 1969–70 season, and became the youngest ever recipient of the MVP at the age of 21. His 986 field goals made, 1,637 rebounds, and 19.5 rebound per game average are the all-time ABA records for a season. Haywood also won the ABA's 1970 All-Star Game MVP that year after recording 23 points, 19 rebounds, and 7 blocked shots for the West team.
Haywood was named to the All-NBA First Team in 1972 and 1973 and the All-NBA Second Team in 1974 and 1975. Haywood's 29.2 points per game in the 1972–73 season and 13.4 rebounds per game in 1973–74 are still the single-season record averages for the SuperSonics for these categories. Haywood played in four NBA All-Star Games while with Seattle, including a strong 23 point 11 rebound performance in 1974. In the 1974–75 season, he helped lead the SuperSonics to their first playoff berth. Overall, during his five seasons with Seattle, Haywood averaged 24.9 points per game and 12.1 rebounds per game.
During the late 1970s, Haywood became addicted to cocaine. He was dismissed from the Lakers by then-coach Paul Westhead after Game 1 of the 1980 NBA Finals for falling asleep during a practice due to his addiction.