Westphal was chosen in 1972 by the Boston Celtics. After three seasons there, he was traded to the Phoenix Suns, team that he helped get to the 1976 NBA finals. In Game 5 of that series, called by many the greatest game ever played in basketball, Westphal made a critical play: With one second to go and the Suns down 111 to 110 but in possesion of the ball and trapped by the Celtics' defense, Westphal called for a time-out. He knew the Suns had no time-outs left, so a technical foul was called on him. The Celtics made a free throw, taking a two point lead, but the Suns had a chance at inbounding the ball without being trapped, and then, Garfield Heard made a shot for the Suns that sent the game into triple overtime. The Suns ended up losing the game and the series.
After the 1979-1980 season, he was traded again, to the Supersonics, where he played one season before heading to the New York Knicks. In 1983, he returned to Phoenix for a last hurrah. Injured, he only played in 59 of the 82 games of his final season.
He scored a total of 12,809 NBA points for an average of 15.1 points per game, with 3,591 assists for a total of 4.4 assists per game. He also had 1,580 rebounds, for a game average of 1.9 rebounds per game.
In 1992 he resurfaced in the NBA, as coach of the Phoenix Suns. With players such as Charles Barkley, Kevin Johnson, Dan Majerle and Danny Ainge, the Suns made it to the finals on Westphal's first season as a coach, but they lost to the Chicago Bulls in six games.
The Suns did not make it back to the finals with Westphal as coach, however, and he went on to coach a high school team in California. But he eventually returned to the NBA as a coach with the Supersonics in the late 1990s. He coached in Seattle until he was fired during the 2000-01 season. He returned to the college ranks in the fall of 2001 at Pepperdine.
In the book Tough Enough: How the Suns Conquered the West (1993), Barkley was quoted saying He (Westphal) can't worry about being our friend, we need a friend, we get a dog!. Barkley was, apparently, hinting that he liked Westphal's way of coaching the Suns.