Westphal was born in Torrance, California. He went to Aviation High School and then USC. He was the 10th overall pick in the 1972 NBA Draft by the Boston Celtics. After three seasons in Boston, including a championship ring in 1974, he was traded to the Phoenix Suns. In 1976, Westphal helped the Suns reach their first-ever Finals appearance, where they played the Celtics. In Game 5 of that series, called by many[who?] the greatest game ever played in basketball, Westphal made several memorable plays:
With time winding down in regulation and the Suns trailing 94–91, Westphal poked the ball away from Jo Jo White, took a long pass from Dennis Awtrey, and hit a breakaway layup. He was fouled on the play and hit the free throw to tie it at 94–94.
In the second overtime, with 15 seconds left and the Suns trailing 109–108 after just making a basket, Westphal stole the ball from John Havlicek after Havlicek received the inbounds pass. This began a chain of events that culminated with Curtis Perry hitting a jumper to give the Suns a 110–109 lead.
After the Perry basket and a Celtics timeout, Havlicek got the ball with five seconds left and hit a bank shot to put the Celtics up 111–110. The buzzer sounded and Celtic fans poured on the floor, thinking the Celtics had won. However, referee Richie Powers correctly ruled that Havlicek's shot went through the basket with two seconds left on the clock. After one second was placed back on, Westphal, sensing that the Suns could get a better shot off if the ball was inbounded at halfcourt, called a timeout that the Suns didn't have while preparing to inbound the ball from under the Celtics' basket. A technical foul was assessed; the Celtics made the subsequent free throw and increased their lead to 112–110. The resulting timeout call allowed Phoenix to inbound the ball at midcourt, rather than go the full length of the court. (As a result of this play, the NBA changed the rules prior to the following season.) Garfield Heard then made a shot for the Suns that sent the game into triple-overtime.
With 20 seconds left in the third overtime and the Celtics seemingly in control at 128–122, Westphal scored two quick baskets to cut it to 128–126 and nearly made a steal at midcourt after the second, but failed and the Celtics ran out the clock to the victory.
Westphal was 6th in the NBA in scoring average for the 1977–78 season at 25.2 ppg. The following 1978–79 season, he was 7th with a 24.0 points per game average.
In his career Westphal scored a total of 12,809 NBA points for an average of 15.1 points per game, with 3,591 assists for an average of 4.4 assists per game. He also had 1,580 rebounds, for an average of 1.9 per game. He was a 5-time All-Star and 3 times an All-NBA selection and one time a second team All-NBA selection. He is Phoenix's fifth all-time leading scorer (9,564), averaging 20.6 points (1975–80, 1983–84) and a member of the Sun's Ring of Honor.
While the Suns made the playoffs during each of Westphal's seasons as coach, they did not return to the finals, and Westphal was replaced during the 1995–96 season. He served as an assistant coach for a high school team in Arizona for two years before he returned to the NBA as a coach with the SuperSonics for the 1998–99 season. He coached in Seattle until he was fired during the 2000–01 season.
He returned to the college ranks in April, 2001 at Pepperdine University. In his first season, Westphal led the Waves men's basketball team to a 22–9 record and tied nationally ranked Gonzaga for the WCC title. The team achieved an at-large berth to the NCAA Tournament, but lost 83–74 to Wake Forest in the first round in a game played at Arco Arena. This was the only postseason berth during the rest of Westphal's five-year tenure and he finished with an overall record of 74–72. After a 7–20 season in 2005–06, Westphal was fired on March 15, 2006.