Riley was a bench player in the early 1970s with the New York Knicks, team that he helped to the 1973 NBA world title. After a relatively obscure NBA career as a player, he retired, but he returned to the NBA, in the early 1980s, as an assistant coach with the Los Angeles Lakers.
Riley was ascended to head coach in 1982, and the team defeated the Philadelphia 76ers for the championship that year. Both teams returned to the NBA Finals in 1983, but Riley's Lakers were swept by the 76ers that time. Riley would experience NBA Finals defeat again in 1984, when his Lakers lost in seven games to the Boston Celtics. In 1985, however, Riley earned his second title as a coach, when the Lakers beat the Celtics in six games in the NBA Finals. In 1986, the Lakers lost in five games in the Western Conference Finals to the Houston Rockets, Ralph Sampson's now famous buzzer-beater shot sending the Lakers into elimination in Game 5.
Riley's 1987 Lakers team is considered one of the greatest teams of all times by many. Aided by Laker regulars Magic Johnson, James Worthy and Kareem Abdul Jabbar, plus role players such as Michael Cooper and Byron Scott, the Lakers regained the NBA championship, once again beating the Celtics in six games.
One of Riley's most famous moments came during the championship celebration, when he promised the crowd a repeat during the Lakers' championship parade in downtown Los Angeles. While 1988's Lakers did not produce as many wins as the team's 1987 version, nevertheless, they became the first team in 20 years to repeat as champions, making good on Riley's promise by beating the Detroit Pistons in seven games in the 1988 NBA Finals. But in 1989, the Lakers, plagued by injuries, lost to the Pistons when the Pistons swept them in an NBA Finals rematch.
Riley stepped down as coach of the Lakers after they lost to the Phoenix Suns in five games at the 1990 NBA playoffs, amid rumors of player mistreatment and anger problems by his part. Despite the rumors and his resigning, he was named NBA coach of the year for the first time after that season. Riley accepted a job as a television commentator after leaving the Lakers, but this job only lasted one year, because he was signed by the Knicks in 1991 as head coach. In 1993, he received his second coach of the year award. Riley returned to the NBA Finals in 1994, but his Knicks lost in seven games to the Rockets. In 1995, Riley resigned, and went on to coach the Miami Heat. In 1997, Riley was selected as coach of the year for the third time.
Riley was so disgusted with the Heat's performance in 2002 that he declared that he was about to fire himself during one stretch of that season. Before the beginning of the 2003-2004 season, he did step down as Heat coach, to dedicate himself to his job as general manager of the team full-time.
Outside of basketball, Riley is known for his friendship with Giorgio Armani, prefering to wear Armani suits during basketball games, and even modeling at an Armani show once.
Riley is second in the all-time winningest coach list, behind Lenny Wilkens.