Early years

Although Kelly initially was interested in playing volleyball and not basketball at high school, soon she joined her sister Coco and they made it to a championship in New York, where they lost in the finals. They also helped their school go 27-0 and win the Minnesota state's class 4A championship. Miller was named a WBCA All-American. She participated in the WBCA High School All-America Game where she scored thirteen points.

College years

The twins went to University of Georgia, where they both majored in biology and won a series of awards, including the James E. Sullivan Award, given to the nation's top amateur athlete. They earned that award in 1999, becoming the first pair of twins to earn the award, and joining such luminaries as Carl Lewis, Greg Louganis, Bill Walton, Bill Bradley, Kurt Thomas, Jackie Joyner-Kersee and Janet Evans as recipients of the award.

Kelly ranked second in points among University of Georgia women players with 2,177; second in assists with 639; fourth in steals with 379 and tenth in rebounds with 711. She is the only player in UGA women's basketball to rank among the top ten in points, assists, steals and rebounds. She became the third player in that university's history to pass over 2,000 points, and the third player in the Southeastern Conference's (SEC) history to garner "Player of the Week" awards three weeks in a row.

WNBA career

In 2001 both Kelly and Coco both entered the WNBA Draft. Kelly was selected by the Charlotte Sting 2nd overall in the 1st round, where she averaged 4.6 points, 1.7 rebounds and 1.1 assists per game in her rookie year.

Kelly spent three seasons with the Sting as a utility player before she was traded to the Indiana Fever prior to the 2004 season. It was with the Fever that Kelly finally became a first-string player on the team, starting on all 34 games she played in the 2004 season.

After the 2005 season ended, Kelly was traded to the Phoenix Mercury, in exchange for Anna DeForge.

On January 30, 2009 Miller was traded along with LaToya Pringle to the Minnesota Lynx for Nicole Ohlde.