Boyd signed a senior contract with Kilmarnock at the age of sixteen on 25 August 1999, and made his debut for the club as a substitute on the last day of the 2000–01 season against Celtic. He was given his chance to stake a place in the team the following season with the departures of Ally McCoist (who finished his career the day of Boyd’s debut) and Christophe Cocard, scoring four goals in total.

Boyd started the 2002–03 season, in “impressive” form and won the SPL Young Player of the Month award for August. In total he scored 12 times in the season and won Kilmarnock’s Young Player of the Year award. His form attracted the interest of Wolverhampton Wanderers, and he had a trial with the Molineux club in August 2003. Boyd scored a total of fifteen goals in the 2003–04 season. In September 2004, he equalled a Scottish Premier League record when he scored all five of Kilmarnock’s goals against Dundee United. He scored 19 goals in all competitions in the 2004–05 season, and was then linked with a move to Aberdeen.

Following speculation that he would sign in the January transfer window, Boyd officially joined Rangers on 1 January 2006. He was signed for a fee of around £500,000 on a contract lasting until the summer of 2009. He waived half of his £40,000 signing on fee, which Kilmarnock were due to pay him under the terms of his contract, to help fund the youth setup at Kilmarnock.

He made his debut on 7 January 2006 against Peterhead in the Scottish Cup third round, in which he scored a hat-trick during a 5–0 win. He went on to score 20 goals in 17 starts for Rangers in the second half of that season, ending it with a total of 37 goals for Rangers and Kilmarnock. He became the first player to finish top scorer at two clubs in one season, having scored 17 goals for Kilmarnock before his move.

On 11 May 2006, he made his debut for the Scotland national team and scored twice in a 5–1 win over Bulgaria in the Kirin Cup. Boyd then took part in European Championship qualifying, scoring twice against the Faroe Islands in a 6–0 win and also against Georgia in a 2–1 win, his fifth goal in six games.

After scoring a penalty against Motherwell in January 2007, Boyd was involved in controversy when he held up six fingers, reportedly in a show of solidarity for former club captain Barry Ferguson (who wore the number six shirt), who had been stripped of his position and dropped from the team following a dispute with Paul Le Guen. Le Guen left the club days later and was replaced by the then Scotland manager Walter Smith. By the end of that season, Boyd scored 25 goals for Rangers in all competitions. Boyd scored his 100th Scottish Premier League goal, and his first against Celtic on 5 May 2007, in a 2–0 victory for Rangers.

The 2007–08 season saw Boyd score his 50th and 51st goals in all competitions for Rangers in a League Cup match against East Fife on 26 September 2007, 627 days after his debut. This made him the second fastest Rangers player ever to reach 50 goals for the club, behind Jim Forrest.

Boyd won the first trophy of his career on 16 March 2008 when Rangers defeated Dundee United to win the 2008 League Cup. He scored both of Rangers’ goals in the 2–2 draw and scored the winning penalty in the subsequent penalty shootout. He also scored a double in the 2008 Scottish Cup Final, a 3–2 win over Queen of the South. He finished that season with 25 goals in all competitions for Rangers.

On 11 October 2008, Boyd stated that he would not play international football again under manager George Burley. His decision came after Burley had left Boyd on the substitute bench in a 0–0 World Cup qualifying match against Norway, opting to bring on debutant Chris Iwelumo instead. When asked about the decision, Burley stated that Boyd had “to establish himself in the Rangers team, which he hasn’t done”. After hearing those comments, Boyd contacted Burley to inform him of his decision, and confirmed his decision with Gordon Smith, the chief executive of the Scottish Football Association. At a press conference on 13 October 2008, Burley hinted that the door might not be closed should Boyd reconsider, stating that Boyd, rather than he, had “made the decision”, and that “… if [Boyd] is totally committed, that’s what the country needs”. On 16 November 2009, Burley was sacked as manager of Scotland after winning just three out of fourteen games as manager.

In January 2009, speculation surfaced about a possible move to Birmingham City to play for former Rangers manager, Alex McLeish. Rangers accepted a bid of nearly £4 million; however, Boyd could not agree personal terms and decided to stay at Rangers. In season 2008–09 Boyd won his first Scottish Premier League title and another Scottish Cup. He finished as top scorer again for Rangers with 31 goals in all competitions, his highest total yet, and was awarded the club’s first Sam English Bowl for top Rangers scorer in a season.

On 30 December 2009, Boyd scored five goals in a 7–1 victory over Dundee United; in doing so, he passed Henrik Larsson as the all-time leader in goals in the Scottish Premier League. On 1 May 2010, Boyd scored his 100th league goal for Rangers at Tannadice against Dundee United.

After Craig Levein was appointed manager of Scotland in December 2009, Boyd stated that he was “ready and willing” to play for Scotland again. He was called up to the Scotland squad to play against the Czech Republic in a friendly on 3 March, and he received his sixteenth cap in the second half of the game. His most recent international appearance was in a Euro 2012 qualifier against Liechtenstein on 7 September 2010. Boyd was recalled to the squad for a friendly match with Norway in November 2013, but he did not play.

In the 2009–10 season, Boyd was the top scorer with Rangers and the Scottish Premier League again with 26 goals in 40 games. With this return he won the Sam English Bowl for the second time and the Scottish Premier League and League Cup both for the second time. On 28 May 2010, Rangers admitted that Boyd would be leaving once his contract runs out.

On 5 July 2010, Boyd signed a two-year contract with Championship side Middlesbrough, with a weekly wage estimated to be around £30,000. On 22 August 2010, he scored his first goal for Middlesbrough in a 1–0 victory against Sheffield United. His second followed on 28 September against Derby County. Boyd scored in a 2–1 defeat against Leeds United on 16 October 2010, which was Gordon Strachan’s final game in charge of the club. He scored his fourth of the season against Bristol City in Tony Mowbray’s first game as Boro boss. On 9 November 2010, Boyd scored the opener in Middlesbrough’s 2–0 win over Scunthorpe. Boyd achieved his 200th club career goal on 1 February 2011, in scoring the first in Middlesbrough’s 2–0 home victory over Scunthorpe by latching onto a long ball from Seb Hines before lobbing the goalkeeper. On 8 March 2011, Boyd joined Nottingham Forest on loan until the end of the 2010–11 season.

Boyd signed a three-year contract with Turkish Süper Lig club Eskişehirspor on 8 July 2011.

Boyd terminated his contract on 19 December 2011 and started legal proceedings against Eskişehirspor claiming he had never been paid during his stay at the club. He played just 76 minutes of league football for Eskişehirspor.

On 23 January 2012, Boyd declined a contract offer from Major League Soccer side Houston Dynamo. Boyd instead signed with Portland Timbers of MLS on 30 January 2012, becoming the club’s second Designated Player. As Houston held exclusive rights within MLS to negotiate with Boyd, Portland were forced to trade a first-round pick in the 2013 MLS SuperDraft to the Dynamo for the right to sign him.

Boyd scored his first goal for the Timbers on 4 March 2012 during a pre-season exhibition against AIK of Sweden. Eight days later, Boyd made his league debut in their season opener, where he scored Timbers’ second goal of a 3–1 victory against Philadelphia Union. Boyd featured less frequently, however, after head coach John Spencer was fired. He scored seven goals in 26 appearances for Portland in the 2012 Major League Soccer season. On 31 January 2013, the Timbers announced that they had mutually agreed to end Boyd’s contract.

Boyd signed a short-term deal until the end of the 2012–13 season with Kilmarnock in February 2013. Boyd was given the number eleven shirt. In 2013–14, Boyd went on to have his most prolific season for Kilmarnock, scoring 22 goals in 37 matches. His success in front of goal garnered a Scotland recall and a Player of the Year nomination.

Boyd signed a one-year contract with Rangers, of the Scottish Championship, on 27 June 2014. He made his first competitive appearance of his second spell at Ibrox on 5 August 2014; playing in Rangers’ 2–1 win over Hibernian in the first round of the Challenge Cup. A fortnight later, Boyd scored his first goals of the season; netting a hat-trick in an 8–1 rout of Clyde in the second round of the Challenge Cup. Boyd was released by Rangers at the end of the season, along with several other players, after the club failed to gain promotion to the Scottish Premiership.

Boyd signed a three-year contract with Kilmarnock in June 2015. On 13 August 2016, Boyd scored his 100th goal for Kilmarnock, in a 2–1 win away to Hamilton Academical. He scored the fastest goal in Scottish Premiership history after just 10 seconds on 28 January 2016, in a 3–2 home defeat of Ross County. Boyd scored his 200th top flight league goal, in a 2–1 home defeat to Motherwell on 6 March 2017, becoming the fifth post-war player to reach that milestone. In December 2017, Boyd scored both Kilmarnock goals in a 2–1 win against Rangers.

In 2016, Boyd’s younger brother Scott took his own life aged 27 without explanation. 16 months later, the Kris Boyd Charity was established with the aim of assisting persons with mental health problems and to encourage discussion of the issues causing them.

On 6 April 2019, Boyd scored from the penalty-spot in a 2–0 victory over St Johnstone. With the goal, Boyd became the club’s joint-second league goalscorer of all time, level with Eddie Morrison on 121 goals. It was also Boyd’s 300th goal in his professional football career across his various clubs and with Scotland.

He announced his retirement from professional football in June 2019.