Miller grew up in Musselburgh and got his big break when he signed for local side Hibernian, and he made his debut against Motherwell in the 1997–98 season. A loan spell at Stenhousemuir saw Miller score five doubles—10 goals—in 13 league and Scottish Cup games, before he returned to Hibernian to establish himself as a regular player. Highlights of his four years at the club included being the club’s top scorer in season 1999–2000 and receiving the Scottish PFA Young Player of the Year award in 2000.

Miller signed for Rangers at a cost of £2 million on 26 June 2000, signing a five-year contract. He made his debut in a 4–1 win over FBK Kaunas on 26 July during a Champions League qualifier. His first goal came on 5 August 2000 during a league match away to Kilmarnock, scoring the third in a 4–2 win.

Miller made his under-21 debut for Scotland in a 2–0 defeat to France at Rugby Park in March 2000. It was the first of a total of seven appearances at under-21 level, in which Miller registered two goals.

Three months later, Miller netted five goals (a Scottish Premier League record) during a 7–1 win against St Mirren. More goals followed against AS Monaco and Aberdeen and helped him to a tally of ten goals from just fourteen games, all coming before the turn of the year. Chances became limited thereafter and, as the season came to a close, he netted only once more against Brechin City in a Scottish Cup match on 27 January 2001. This proved to be his final strike that season. Miller had made a total of 38 appearances and scored 11 goals for Rangers.

In September 2001, Miller went to Wolverhampton Wanderers on a three-month loan, scoring against Gillingham and Walsall. However, after just five appearances, Miller broke his collarbone in a fall. In December 2001, Wolves secured him on a permanent transfer at a cost of £3 million, on a four-and-a-half-year deal.

Miller made his international debut, under the management of Craig Brown, on 25 April 2001. He was an 80th-minute substitute for Scott Booth during a 1–1 friendly match against Poland. He had to wait nearly two years for his second appearance when he was selected for a Euro 2004 qualifier by the then manager Berti Vogts in March 2003. Miller started the match at home to Iceland and scored within the first ten minutes to give Scotland the lead.

In summer 2005, his form attracted the attention of the newly promoted Premier League team Sunderland, but Wolves rejected their £1m and £1.2m offers and Miller began the new season as a Wolves player. He was subject of further transfer bid in the January transfer window, however Wolves rejected a £1.5m offer for Miller from Sheffield United after the player stated he was not interested in the move. Despite only playing in his natural position on few occasions, Miller still ended the season as top scorer, with 12 goals – the third time in Wolves’ last three Championship campaigns. Miller played 191 games and scored 63 goals during his five years with the club.

Miller was handed the number nine jersey upon his arrival at Celtic. He endured a difficult start at the club, getting sent off in a pre-season friendly with D.C. United and failing to score in his first nine games. He finally netted his first goal, against Old Firm rivals and former employers Rangers at Celtic Park in September 2006. He proceeded to score in Celtic’s 1–0 Champions League win against Copenhagen from the penalty spot and netted two in the 3–0 win over Benfica. By scoring in the 2006–07 competition, Miller became the first player to score goals in the Champions League proper for both Rangers and Celtic. However, he ended the season with only four league goals. He did play in the Scottish Cup final, before being substituted because of an injury. Despite hints from the player himself that he would leave to get more first-team opportunities, Miller vowed to stay at the club and fight for his place in the team. Miller scored his first goal in his first appearance of the 2007–08 season in a 4–1 victory over Falkirk and added to his collection the following week with a brace against Aberdeen. He netted 11 goals in total for the club in 46 appearances.

Miller signed a three-year deal with newly promoted Premier League club Derby County on 31 August 2007 for an undisclosed fee, later confirmed by Derby County manager Paul Jewell to be close to £2.25m. Miller started well at his new club, scoring in his first two home matches, including the winner on his league debut for the Rams against Newcastle United in a 1–0 win on 17 September, a strike which won the club’s Goal of the Season award. However, following this initial success Miller netted just twice in his next 28 league fixtures. After less than seven months at the club and with Derby looking likely candidates for relegation, Miller expressed a desire to move – return to Rangers being a possible destination, although manager Jewell insisted any move would have to be financially fair for Derby.

A fee of £2 million was agreed between Rangers and Derby allowing Miller to enter into discussions about a transfer back to Ibrox on 10 June 2008. The deal was completed on 13 June. Miller’s return means he is the only player in the modern era to cross the Old Firm divide twice, and only the second in history, the first being Tom Dunbar in the 1890s.

Miller made his second debut for the club on 30 July 2008 in a Champions League qualifier against FBK Kaunas, who he played during his first Rangers debut. He netted a brace in the first Old Firm game of the season on 31 August 2008. He followed this up with a further two goals against former employers Hibernian at Easter Road a month later. Miller played in the League Cup Final in March 2009 but Rangers lost the match 2–0 to Celtic after extra time. However, he won a league winners medal with Rangers on the final day of the season and followed it up with a Scottish Cup winners medal.

Miller had an injury-ridden start to the 2009–10 season, but gave a Man of the Match performance in the first Old Firm game of the season, scoring both of Rangers’ goals in a 2–1 win. In March 2010, he scored the winning goal in the League Cup Final against St Mirren, after Rangers had been reduced to nine men.

Miller was handed the number 9 shirt after Kris Boyd moved to Middlesbrough. He scored his second Rangers hat-trick against former club Hibernian on 22 August 2010; his first was when he scored five against St Mirren in November 2000. Miller claimed to be in “the form of his career” after notching up ten goals in the first six league matches of the 2010–11 season. Miller scored twice in the first Old Firm derby of the season.

Miller won his 50th cap for Scotland in a 1–0 defeat against Czech Republic in October 2010. He captained the team for the first time, and scored, against Northern Ireland on 9 February 2011. Miller continued to represent Scotland after moving to Major League Soccer club Vancouver Whitecaps FC. He scored in the 3–2 defeat against England in August 2013, which meant that Miller had scored 18 goals in 69 appearances for Scotland. Miller retired from international football soon after the England match, with Scotland manager Gordon Strachan stating that Miller wanted to prolong his club career.

On 21 January 2011, Miller officially signed for Bursaspor for £400,000 on a two-and-a-half-year contract. He made his debut for the Turkish side two days later in a goalless draw with Konyaspor when he played the last 25 minutes after replacing Pablo Batalla. Kenny scored on his first start for Bursaspor in a 2–0 win over Galatasaray. He followed this up with a further two goals in his next game against Sivasspor, netting an 88th-minute winner. Miller scored five goals in 15 appearances for the club.

Miller signed for Cardiff City on 26 July 2011, for a reported transfer fee of £870,000. Miller scored his first goal in the 91st minute of his debut match against West Ham United in the season opener. Miller returned to the score sheet, on 28 September, with a brace in a 2–1 win over Southampton. However, Miller suffered a groin injury in 10 minutes the following game against Hull City, which kept him out of two Scotland games and two Championship matches, where Cardiff only gained a point. On 19 October, manager Malky Mackay confirmed that Miller was fit and ready to make a return against Barnsley. Miller did return this match and scored the opener, but only lasted 30 minutes in the match after suffering a head injury. Miller scored in 1–0 victories against Birmingham City and Nottingham Forest in December. Cardiff reached the 2012 Football League Cup Final, but Miller missed a chance to score late in extra time. He then missed his penalty kick as Cardiff lost to Liverpool in a penalty shootout. He struggled for form later in the 2011–12 season, scoring only once in 22 appearances.

Miller signed with Vancouver Whitecaps FC of Major League Soccer on 16 July 2012. Six days later, he made his debut in a 2–1 home win over the San Jose Earthquakes, playing the last 12 minutes in place of Darren Mattocks. He scored his first goal for the Whitecaps on 25 August 2012 against the Portland Timbers in a 2–1 loss. He signed an extension in July 2013 to take him through June 2014 with the club, however, the two sides mutually agreed to terminate the remaining two months in his contract on 4 May 2014.

From then until his international retirement in 2013, Miller was almost always first-choice, playing either on his own or as part of a two-man attack. He scored goals in important games against several teams, including Germany and Ukraine. In 2005, Miller scored four goals in three internationals, against Austria, Italy, and Norway.

On 4 June 2014, Miller signed for Rangers on a one-year deal. He made his third debut against Hibernian in a Challenge Cup tie on 5 August and scored his first goal since his return to the club on 30 August against Queen of the South. Miller scored nine times in 41 appearances in 2014–15, as Rangers failed to win promotion to the top flight.

Miller signed another one-year extension with Rangers in November 2015. On 2 January 2016, Miller became Rangers’ oldest ever player to score a hat-trick in a 6–0 win over Dumbarton. Five goals in four league games during January 2016 saw Miller win the Scottish Championship player of the month award. He played and scored in the 2016 Scottish Challenge Cup Final, as Rangers beat Peterhead 4–0, only five days after Rangers had secured the Scottish Championship trophy. Miller ended the 2015–16 season with 21 goals, scoring his final goal in the 2016 Scottish Cup Final which Rangers lost 3–2 to Hibernian, giving Hibernian their first Scottish Cup win since the 1901–02 Scottish Cup.

During the 2016–17 season, Miller scored his 100th goal for Rangers, over three spells, in a Scottish Premiership match against Inverness CT. It was strike which was nominated for the SPFL’s and the club’s goal of the month awards for October, winning the latter. He also scored his tenth career goal in an Old Firm derby (nine of them for Rangers) when he captained the side in a 2–1 home defeat to Celtic on Hogmanay 2016. He also scored the only goal for Rangers in a 5–1 defeat to Celtic, the heaviest defeat at Ibrox since 1897. Miller scored his first goals of 2017 with a late brace in a 2–1 Scottish Cup win at home to Motherwell on 21 January and followed this a week later by netting the opening goal in a league match, also against Motherwell. On 28 April, Miller signed an extension to his contract until 2018 with manager Pedro Caixinha stating he was impressed by Millers professionalism and leadership.

Upon signing a contract extension with Rangers in November 2016, Miller took up a part-time coaching role with their Under-20 team. After leaving Rangers in 2018, Miller had a short stint as player-manager of Livingston. He then continued his playing career with Dundee and Partick Thistle. Miller retired as a player in February 2020 and took a coaching role with Australian club Newcastle Jets. Following the departure of head coach Carl Robinson to Western Sydney Wanderers from Newcastle Jets, Miller followed suit joining Robinson as his assistant.

Miller scored the opening goal in Rangers’ 2017–18 season against Progrès Niederkorn in the Europa League on 29 June 2017. Despite his goal, Rangers went on to lose 2–1 on aggregate, giving Progrès their first ever European win. Miller was dropped from the Rangers first team in September 2017. This came after Pedro Caixinha had criticised some of the team’s Scottish players in a meeting after a 2–0 defeat by Celtic. Caixinha then suggested that a Rangers player had leaked team information to opponents, which led journalists to surmise that Miller had been the source. Caixinha was sacked in late October, and Miller was reinstated to the Rangers team by caretaker manager Graeme Murty; in his first game back, Miller scored two goals in a 3–1 win against Hearts.

In April 2018, Miller and teammate Lee Wallace were suspended by the club pending an internal investigation into an altercation with manager Murty, in the aftermath of the 2017–18 Scottish Cup semi-final which Rangers had lost 4–0 to Celtic. Miller was fined by Rangers and left the club at the end of his contract. Five months after the incident for which they were disciplined, Miller and Lee Wallace won an appeal at a SPFL tribunal against the action taken against them, with the expectation they would make a financial claim against Rangers for the fines imposed. Over his three spells at the club, he made 301 appearances in all competitions, scoring 116 goals.

Miller was appointed player-manager of Scottish Premiership club Livingston in June 2018. He scored his first goal for the club on his third appearance, netting the only goal of a 1–0 victory over Dumbarton on 21 July in the group stage of the 2018–19 Scottish League Cup. He left the role on 19 August, with his departure being an ‘amicable agreement’ following discussions between Miller and the club hierarchy; they would have preferred him to give up playing and focus solely on the managerial role, which he was not prepared to do at that time. Livingston said that they had asked Miller to focus on his tasks as manager, but Miller had been unwilling to end his playing career. The following week, Miller’s former Scotland teammate Gary Holt was appointed his successor at Livingston.

Miller signed a two-year playing contract with Dundee on 29 August 2018. He failed to score in his first seven matches for the club but then scored five in three games, including a hat-trick at home to relegation rivals Hamilton Academical on 5 December. Dundee finished in bottom position in the Premiership table and were relegated to the Scottish Championship, with Miller leaving the club ‘by mutual consent’ during the summer off-season.

On 26 June 2019, Miller signed for Partick Thistle on a one-year contract. Miller, scored his first goal for Thistle in the League Cup v Queen’s Park in a 2–1 win. Miller was released from his Thistle contract in January 2020, having scored 10 goals in all competitions for the club. He announced his retirement from playing football soon afterwards.