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, Mintlaw Primary School and Mintlaw Boys Club 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 three and half hours each way from Aberdeenshire 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, where 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.

In March 2008 at the age of 17, Little joined Arsenal in the FA Women’s Premier League National Division. She scored her first goal for the club in April, during a 4–1 win over Chelsea before 5,000 fans at Emirates Stadium. Arsenal manager Vic Akers told reporters: “Make a note of this kid, she’s going to be a big player.” She was not eligible to play in Arsenal’s 4–1 FA Women’s Cup Final victory over Leeds United on 5 May 2008.

The following season Little’s prolific goal scoring from the midfield helped Arsenal overcome the departure of Kelly Smith. After 47 goals in all competitions, the team finished the regular season in first place with a 20–1–1 record. Little was the league’s leading scorer with 17 goals. She was named FA Players’ Player of the Year in June 2010. Arsenal entered the 2010 FA Women’s Cup Final hoping to lift the trophy for a fifth consecutive year. Despite Little’s first half penalty, they were eventually upset 3–2 by Everton, after extra time.

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.”

During her 50th cap, Little scored the game-opening goal in Scotland’s 2–0 victory against rival England at the Cyprus Cup in March 2011. It was only the second time Scotland had ever defeated their rivals, and was the first time since 1977 – 13 years before Little was born.

During the 2012 season, Little was the league’s top scorer with 11 goals. Arsenal clinched the regular season title with an undefeated 10–4–0 record and claimed their ninth consecutive English title. During a match against Chelsea L.F.C. on 26 April, she scored a brace with goals in the 47th and 90th minutes helping Arsenal win 3–1. She scored another brace during a 2–0 win against Liverpool L.F.C. on 6 May. Little was voted Women’s Players’ Player of the Year for 2012–13 by the Professional Footballers’ Association, in the first year that the award was given to women. Upon receiving the award, Little said, “For years, I have watched this event on television, so to come here and win the first award is fantastic.”

On 16 June 2012, Little scored Scotland’s first hat trick during the team’s 8–0 win against Israel. In October 2012, she scored in both legs of Scotland’s UEFA Women’s Euro 2013 qualifying play-off against Spain. In the second leg Verónica Boquete scored in injury time of extra time to make the score 3–2 to Spain and eliminate the Scots, who were left “utterly disconsolate” at missing out on qualification for their first ever final tournament.

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.”

In November 2013, Little left deposed champions Arsenal after six years for American National Women’s Soccer League (NWSL) club Seattle Reign FC. Laura Harvey had departed Arsenal to coach Seattle Reign the previous year and had made Little her top transfer target. Of the signing, Harvey said, “Kim is world-class. Her talent and proven experience will be a huge asset for our team in the coming seasons. Having coached Kim for nearly four years, I have no doubt that her creativity and goal-scoring ability will prove an exciting proposition for the NWSL.”

In February 2013, Little scored Scotland’s only goal during a friendly match against the United States in which the U.S. won 4–1. During the 2013 Cyprus Cup, Little scored a goal and served an assist in Scotland’s 4–4 draw against England during the group stage of the tournament. She scored another goal in the 11th minute of the team’s fifth place match against the Netherlands resulting in a 1–0 win for Scotland.

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 17 October 2016, Little announced that she would return to Arsenal L.F.C ahead of the 2017–18 FA WSL season. Harvey said Little was given an “incredible offer” of a multi-year contract, though no other details were disclosed.

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..

In May 2017, Little sustained a rupture to her cruciate ligament during training with Arsenal, missing the better part of the season. On 14 March 2018, in the FA WSL Continental Tyres Cup final, Arsenal defeated Manchester City 1–0 to claim their fifth title.

Little suffered a fracture to a fibula during a 5–0 win against Chelsea in October 2018. The match officials were criticised for failing to send off Chelsea’s Drew Spence, whose “heavy tackle” had injured Little. It was estimated that the injury would prevent her from playing for 10 weeks. Arsenal won the 2018–19 Super League, their first league championship in seven years.

Little signed a new contract with Arsenal in August 2019. She underwent surgery on an injured foot in February 2020.

Little missed UEFA Women’s Euro 2017 due to her serious injury. After recovering from the injury, she helped Scotland qualify for the 2019 FIFA Women’s World Cup, their first appearance in a world finals. Little scored the only goal in a friendly against Brazil in April 2019. At 2019 World Cup, she scored the opening goal of the 3–3 draw with Argentina.