Kelly Griffin was born and raised in Berkeley, California and played soccer and basketball starting at the age of 6. She attended Berkeley High School in Berkeley, California, where she played basketball for coach Gene Nakamura, who called Griffin's 2003 team the group he was "most fond of" from his 24 seasons as head coach.

Griffin moved from Berkeley to Los Angeles to attend UCLA in 2004, and her freshman she year joined the UCLA Women's Club Rugby team, known as the UCLA Bruin Rangers. She helped lead UCLA to several national playoff appearances, and was named a USA Rugby AIG Women’s Collegiate All-American in 2007 and 2008. Additionally she was selected for the USA Rugby Under-23 team in 2006 and 2007.

After graduating from UCLA with an applied math degree in 2008, Griffin returned to the San Francisco Bay Area and joined the Berkeley All Blues Women's Rugby Club. During the 2011 season of the Women's Premier League, Griffin won the 7s National Championship with the All Blues and was named tournament Most Valued Player. The All Blues also won the 2011 Women's Premier League Championship, with Griffin leading the league in tries scored. Additionally, Griffin won the 2011 Women's All-Star National Championship with Pacific Coast team, rounding out her three championships in a six-month period.

In early 2012, Griffin signed a full-time training contract with USA Rugby, and along with 7 other women, became the first professional female rugby players in the United States. Griffin moved to Chula Vista, California to train at the Olympic Training Center with the rest of the contracted 7s players. From 2011/2012 on, Griffin has played in the World Rugby Women's Sevens Series, captaining the USA team for several tournaments. Additionally, going into the 2016 Olympic Games, Griffin leads the country in appearances on the World Sevens Series Circuit, and is the only athlete of the "original eight" to have remained in residency throughout the four-year lead up to the 2016 Olympics.

In addition to the World Rugby Women's Sevens Series, Griffin competed with the USA Eagles Sevens at the 2013 Rugby World Cup Sevens. After an undefeated campaign in pool play, the USA went on to claim the bronze medal by beating Spain in sudden death overtime 10-5 in the 3rd place match.

In July 2015, Griffin competed as part of Team USA at the 2015 Pan American Games in Toronto. The United States women's 7s team lost to Canada in the gold medal match, leaving the tournament with sliver.

On July 18, 2016, Griffin was named as a member of the USA Women's Olympic Rugby Team, which will compete in the Rio 2016 Olympics, the first Olympics Games to include women's rugby. Griffin was later announced as the team's captain for the Olympics Games.