Halard-Decugis lived in La Baule, France during the initial stages of her career and later moved to Pully, Switzerland. She turned professional in June 1987 after winning the French Openjunior singles title in 1986 and 1988 and reaching the Wimbledon junior singles final in 1987. 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 been coached by Arnaud Decugis since 1989.
Halard-Decugis won her first WTA Tour singles title in Puerto Rico. She enjoyed her best season in 1996, when she won her first WTA Tour Tier II singles title 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 played two tournaments in late 1997 because of injuries.
By winning the singles title in Rosmalen in 1998, she became the 20th player to have won singles titles on all four surfaces in the Open Era. Halard also won the singles and doubles titles in Pattaya that year, and broke into the top 10 singles ranking in August 1999, becoming the fifth Frenchwoman after Françoise Dürr, 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 other occasions. Between November 15, 1999 and January 9, 2000, Halard-Decugis, Nathalie Tauziat,Amélie Mauresmo and Mary Pierce were all ranked inside 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 Halard's professional playing career. She reached the Australian Open singles quarter-final for the second time, 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. Halard was also runner-up in Tokyo's Princess Cup in the month of October and won the doubles title with Ai Sugiyama. The following week, she won both the singles and doubles titles at the Japan Open in Tokyo, saving three match points in the final to defeat the defending champion Amy Frazier.
On her 30th birthday, Halard won the 2000 US Open women's doubles title with Ai Sugiyama, her only Grand Slam title as a professional. The pair also reached the final at Wimbledon, the semi-final at the French Open and the quarter-final at the Australian Open that year. Halard-Decugis won nine other doubles titles in 2000, five of them with Sugiyama, and became the first Frenchwoman to attain the number one WTA Tour doubles ranking in the Open Era.
Halard-Decugis represented her country in the Federation Cup Fed Cup from 1990 to 2000 and in the Olympics Games in 1992 and 2000.
She married her coach, Arnaud Decugis, on September 22, 1995. Arnaud Decugis is the great nephew of Max Decugis, a leading tennis player from France during the early 20th century. The couple have 2 children: Camille, born February 10, 2002 and another child born in July 2003.