Halard-Decugis played right-handed, had a height of 1.73m and weighed 57 kg in 1997. She lived in La Baule, France during the initial stages of her career and later moved to Pully, Switzerland. She turned professional in June 1987. She showed a lot of potential in her teenage years by wining the French Open junior singles title in 1986 and 1988 and by being the junior singles runner-up in the 1987 Wimbledon tournament. She retired from the WTA Tour tennis circuit at the end of the 2000 season. Her highest WTA Tour singles and doubles rankings was number seven and number one respectively. She had career official prize money earnings of US$3,096,734. Her final career singles win-loss record was @386-233 and her final career doubles win-loss record @253-156 (@includes main draw and qualifying matches in all WTA Tour//ITF Tour tournaments, Federation/Fed Cup and Olympic Games). She had been coached by Arnaud Decugis since 1989.

Halard-Decugis won her first WTA Tour singles title in Puerto Rico. She enjoyed her best season to date in 1996, when she won her first WTA Tour Tier II singles title in her home country in Paris and finished the year with a career-high season-ending singles ranking of number 15 and as the number one singles player from France. This occurred despite the fact that her playing schedule in the second half of 1996 was curtailed because of a wrist injury sustained during the Fed Cup semi-final match against Spain. She only managed to play two tournaments in late 1997 because she had been nursing her wrist and knee injuries for much of the past twelve months.

1998 was to be her comeback year. By winning the singles title in Rosmalen, she became the 20th player to have won singles titles on all four surfaces in the Open Era. She won the singles and doubles titles in Pattaya, the first time she had done so in her professional career. She broke into the top 10 singles ranking in August 1999, becoming the fifth Frenchwoman after Françoise Durr, Mary Pierce, Nathalie Tauziat and Sandrine Testud to do so. In 1999, she won two WTA Tour singles titles and was runner-up on three occasions. Between November 15, 1999 and January 9, 2000, Halard-Decugis,Nathalie Tauziat, Amélie Mauresmo and Mary Pierce were all ranked among the singles Top 10, the first time France had four players ranked among the singles Top 10.

2000 was to be the final and perhaps the finest year of her professional playing career. She reached the Australian Open singles quarter-final for the second time; it was only her third Grand Slam singles quarter-final. She captured the second WTA Tour Tier II title of her career in Eastbourne and reached her career-high singles ranking of number 7 in February. She enjoyed a tremendous two weeks in Tokyo in the month of October, reaching the second singles final of the year at Tokyo after leading former world No. 1 Monica Seles in the semi-final when Seles retired, before falling in the final to No. 2 seed Serena Williams, and winning the doubles title with Ai Sugiyama. The following week, she won both the singles and doubles titles at Tokyo , saving three match points in the final to defeat the defending champion Amy Frazier. She also excelled in doubles play in 2000. She won US Open women's doubles title with Ai Sugiyama, her only Grand Slam title, excluding the junior titles that she won in the 1980s. The championship match was contested on Halard-Decugis's 30th birthday. In addition to winning the US Open, the pair also reached the final at Wimbledon, the semi-final at the French Open and the quarter-final at the Australian Open. Halard-Decugis also won nine other doubles titles in 2000, five of them with Sugiyama. She became the first Frenchwoman to attain the number one WTA Tour doubles ranking in the Open Era as a result of her overall excellent and consistent performances in doubles competition over the past 12 months. As of year-end 2005, she was the only Frenchwoman in the Open Era to reach the WTA Tour women's doubles number one ranking.

In her career, Decugis-Halard has won 12 WTA Tour singles and 15 WTA Tour doubles titles. She was the runner-up in WTA Tour singles and doubles tournaments on 9 occasions each.

In addition to her 2000 US Open women's doubles titles, Halard-Decugis also reached the women's doubles QF or better in 7 Grand Slam tournaments. She was a WTA Tour doubles semi-finalist on 19 occasions, excluding Grand Slams: 1991(1), 1992(2), 1993(3), 1994(2), 1995(3), 1998(4), 1999(3), 2000(1)

Halard-Decugis represented her country in the Federation/Fed Cup from 1990 to 2000 and in the Olympics Games in 1992 and 2000.