Born in Aberdeen, Scotland and raised in Mintlaw, Aberdeenshire, Little began playing football at a young age with her father and brother. She played football for her primary school and local boys teams as a youth. From age 10 to 14, she played at the youth level for Buchan Girls before joining the Hibernian Girls in 2005. At age 13, she attended and played for Mintlaw Academy. During her time with Hibernian Girls, Little would travel about two and half hours each way from Aberdeen to Edinburgh for training. Of her youth, Little said, "Football gave me opportunities from a young age. Having the chance to travel, to play in different environments and to come up against really good players from all over Europe was fantastic and made me more determined to keep playing." Little attended the University of Hertfordshire when she obtained a degree in Sports Studies while playing for Arsenal L.F.C..

At the age of 16, Little made her debut for Hibernian L.F.C. of the Scottish Women's Premier League (SWPL) during a UEFA Women's Champions League match against RCD Espanyol at Almondvale Stadium on 8 August 2006. Hibs had signed Little and other players from their youth team to replace veterans Pauline Hamill, Nicky Grant and Debbie McWhinnie who all left the club that summer. In her SWPL league debut, she scored a hat trick against Hutchison Vale L.F.C. During her time with Hibernian, Little helped the club win the SWPL championship title, Scottish Cup, and the Premier League Cup. She competed with the team in the UEFA Women's Champions League twice. During the 2006–07 season, Hibernian won every match with Little scoring 55 goals in her 30 appearances for the club. The following season, she scored 33 goals in 18 appearances with the club.

Little made her debut for the Scotland women's national football team in February 2007 at the age of 16 in a match against Japan, coming in as a second-half substitute for Megan Sneddon. National coach Anna Signeul had no hesitation in promoting Little into the senior team at 16: "Kim is an exceptional talent. There's no limit to how far she can progress. She has technique, speed and power, and she reads the game extremely well." She scored her first international goal as Scotland lost to Russia in March 2008.

Little played in every league match during the 2008–09 season bar one, scoring 24 goals. Arsenal finished at the top of the regular season table with a 20–1–1 record. She also played in the 5–0 Premier League Cup Final victory over Doncaster Rovers Belles and scored in the FA Women's Cup Final as Arsenal beat Sunderland 2–1 before almost 25,000 fans at Pride Park in Derby. Little later described 2008–09 as her most memorable season, as Arsenal won a treble despite the departure of several leading players to Women's Professional Soccer (WPS) in America.

In 2011, the FA WSL was formed replacing the FA Women's Premier League as the top division of women's football in England. During the 2011 season, Little was the second-leading scorer in the league with nine goals. She scored a brace during a match against Doncaster Belles on 7 May 2011. Arsenal finished at the top of the league table with a 10–2–2 record. At the 2011 FA Women's Cup Final, Little was named Player of the Match and scored the opening goal in Arsenal's 2–0 win over Bristol Academy. After the game Arsenal manager Laura Harvey described Little as "world class."

Despite opposition from Scottish Football Association with regard to Scottish players playing on a British team, Little was strongly in favour of participating in the Great Britain team at the 2012 London Olympics: "I don't see why anyone would want to stop a player from playing at a massive tournament like the Olympics, it's the biggest sporting event ever. If I get the opportunity I'll grab it with both hands – I would definitely play."

Little typically plays as the advanced midfielder in Laura Harvey's 4–3–3 formation, but occupies a deeper role when playing for Scotland. In an interview with FIFA.com in July 2012, Little said: "Nearly all my goals come from midfield. That’s my natural position. But I’m a player who always tries to make forward runs, be positive and to create and score goals."

Little confirmed in an interview in 2014 that she had been in a long-term relationship with professional footballer Tom Pett. In 2016, she appeared in a national campaign for Athlete Ally.

In October 2015, it was announced that Little had signed with Australian W-League team Melbourne City along with Seattle Reign FC teammate Jess Fishlock on loan for the 2015–16 season. On 9 November, she was named the league's Player of the Match after her first start for the club following international duty for Scotland. On 6 December, she served an assist to the first goal of the match by Scottish national teammate Jennifer Beattie and then scored a goal in the 80th minute against Melbourne Victory solidifying City's 4–0 win over their rival and extending City's regular season record to 8–0–0. Little ended the season having played 12 games for Melbourne City and scoring 9 goals, ranking only behind Larissa Crummer. Melbourne City won both the regular season and Grand Final. Little earned Player of the Match Award honours in the Grand Final.

On 4 February, Little announced that she would not return to Melbourne City for the 2016–2017 season, saying her time in the W-League was "a one year thing for me" and citing other priorities in October 2016, which she later announced as her transfer from Seattle Reign FC to Arsenal L.F.C..

Little missed UEFA Women's Euro 2017 due to her serious injury.

On 14 March 2018, in the FA WSL Continental Tyres Cup final, Arsenal W.F.C. defeated Manchester W.F.C. 1–0 to claim their fifth the title.