Up to this point, Roy played almost exclusively with local Chicago minor league teams and the U.S. national team. In 1967, he joined the newly established Chicago Spurs of the NPSL. At the time, the Roy was one of only eight U.S. citizens in the league. One of the other U.S. citizens, Bob Gansler, was later a teammate with Roy on the national team and became the future coach of the national team. In his first year with the Spurs, Roy scored 17 goals, assisted 5 others, making him the league's second leading scorer. This led to his selection to Rookie of the Year and NPSL All Star team.
Once again, external soccer events impacted Roy's career. The NPSL merged with the United Soccer Association in December 1967 to form the North American Soccer League. That year, the Spurs also moved to Kansas City. Roy moved with the team and spent one season there. Roy experienced a sophomore slump in 1968, playing 15 games, scoring 6 goals and assisting on 4 others. However, the team played well, losing to the Atlanta Chiefs in the playoff semifinals.
Roy left the Kansas City Spurs in 1971 to join the Saint Louis Stars of the North American Soccer League (NASL) in 1971. In his three seasons with the Stars, he scored 18 goals and assisted on 16 others. In 1972, the team made it to the NASL championship game, losing to the New York Cosmos 2-1.
After retiring from the national team, Roy continued to play professionally for a few more years. In 1975, he moved to his last team, the expansion Chicago Sting. He would play a single season with the Sting, statistically the worst of his career. He scored no goals and made 3 assists in 14 games. He retired from playing at the end of the season.
Roy enjoyed relative success with the national team in 1968. He played eight games, scoring six goals. In those eight games, the national team went 4-3-1. More significantly, four of those games were qualifying matches for the 1970 FIFA World Cup. In those three games, the U.S. went 3-1 with Roy scoring 3 goals. The next year, he played only a single game with the national team, a 2-0 loss to Haiti. That game, combined with a second loss to Haiti a month later spelled the end to yet another U.S. attempt to qualify for a World Cup. Roy would not play for the national team again until it began playing qualification games for the 1974 World Cup.
In 1972, Roy played four games with the national team, all 1974 World Cup qualifying matches. The U.S. went 0-3-1, failing to reach yet another World Cup. Despite the team's dismal showing, Roy scored in three consecutive games, giving him a record six goals in World Cup qualifying matches. The next player to score in three consecutive matches did not come until Cobi Jones did it in 2000. Roy’s record of six goals in world cup qualifying matches lasted until Earnie Stewart scored his seventh qualifying goal in 2001.
Roy played five more games with the national team, all in 1973. That year, he scored once, ending his national team career with 20 games and 10 goals, one of the best scoring rates by any national team member over a sustained career.
Roy's Sting did not do well in its first season indoors. Things improved during the 1982–1983 indoor season. While the NASL did not hold an indoor season the next winter, the Sting played in the MISL where it finished third in the Eastern Division. Roy and the Sting improved again during the 1983–1984 indoor season, again with the NASL. They finished second in the standings and lost to the New York Cosmos in the playoff semifinals.
The summer of 1984 saw Roy coach the Sting to its second NASL outdoor title. This would also be the last NASL championship as the league folded that fall. With the demise of the NASL, the Sting became a full time member of the MISL.
Roy coached the Sting for two more seasons. The team lost to the Cleveland Force in the 1985 quarterfinals but missed the playoffs in 1986, finishing with a 23-25 record. Roy ending his tenure with the Sting on December 23, 1986.
During his time with the Sting, Roy did more than win two championships. He had an impact which went beyond the team when he coached the former Dutch National Team Head Coach Dick Advocaat and United States women's national soccer team Greg Ryan. Ryan later credited Roy with being a strong influence on his development as a player and coach. Ryan said, “The thing that defined Willy was, he was one of the most intense competitors as a player and that carried over to his coaching. He was so intense and so determined that he brought that quality out in his players. If he mentioned people who went on to bigger and better things, it was because they learned to work so hard and give so much.”
In 1985, Roy became the men's soccer coach at Northern Illinois University. In 1990, the Huskies had the Mid-Continent Conference's best record and Roy was selected as the Mid-Con's Coach of the Year.
On February 18, 2003, the school announced that it was not renewing Roy's contract after the Huskies had three consecutive losing seasons. He finished his career coaching the team with a 142-131-22 record and two conference championships. During his time with the team, it was known for the high academic standards Roy set for his players. Although his last season with the team saw a dismal 4-13-1, the Huskies ranked in the Division-I Academic top 20.
He currently owns and runs the Willy Roy Soccer Dome, an indoor soccer arena in Chicago, and the Dolton Bowl, a bowling alley next door. Now in his sixties, he continues to play in an over-30 league at the Soccer Dome.
In 1979, he wrote the book Coaching Winning Soccer.