She won the Orange Bowl International Junior Championship in Coral Gables, Florida in December 1984.

[2] Nilsmark turned professional in 1987 but her early career was hampered by a whiplash injury sustained in a car accident.

On the Ladies European Tour, she won the Evian Masters in Évian-les-Bains, France, beating Laura Davies, who was at the top of her career, with two strokes.

Nilsmark also won the 1999 Valley of the Stars Championship at the Oakmont Country Club in Glendale, California on the LPGA Tour, beating Annika Sörenstam in a playoff.

Nilsmark played in the Solheim Cup in 1992, 1994, 1996, 1998 and 2000, sinking the winning putt in 1992 in Europe's first victory over the United States.

During her LPGA Tour career, Nilsmark and Hellqvist, who changed his last name to Nilsmark, lived in West Palm Beach, Florida, United States, in a house formerly owned by Tony Jacklin and situated close to were Swedish golfer Jesper Parnevik lived.

In the seventh month of her pregnancy in 1997, she competed in the LET tournament Compaq Open in Stockholm, Sweden, finishing lone runner-up to Annika Sörenstam, ahead of third placed Nancy Lopez.