Turnbull joined the Hartlepool United youth system in 2003 and was a member of the teams enjoying successful Dallas Cup campaigns in 2004 and 2005. He began to establish himself in the reserve team over the next couple of years, while still with the youth team. He joined Northern Premier League Premier Division club Gateshead on loan in December 2005, making his debut in a 0–0 home draw with Radcliffe Borough on 17 December. He made five appearances before returning to Hartlepool in February 2006. He was given his first professional contract with Hartlepool on 4 July 2006. He made his first, and only, appearance for Hartlepool's first team after he started in the 3–1 win over League One team Rotherham United in the Football League Trophy first round on 17 October 2006. He joined Blyth Spartans of the Conference North on 22 January 2007 on a one-month loan and made 11 appearances for the club. He was released by Hartlepool in May 2007.
York City
He was signed by Conference Premier club York City on 6 July 2007. A dislocated shoulder picked up in a pre-season friendly against Frickley Athletic in July 2007 resulted in a six-month lay-off with injury. His York debut came on 12 January 2008 as a 66th-minute substitute in York's 0–0 draw at home to Grays Athletic in the FA Trophy second round.
Gateshead
Having failed to establish himself in the York team under managerColin Walker, Turnbull rejoined Gateshead on a one-month loan in February 2008 and made his debut against Witton Albion. He agreed a permanent contract with Gateshead on 11 March 2008 after he was released by York. He scored his first career goal on 1 April 2008 in Gateshead's 2–0 win over Prescot Cables at Valerie Park. Turnbull finished the season with 18 appearances and 1 goal for Gateshead as they achieved promotion to the Conference North via the play-offs. The team achieved a second successive promotion, this time to the Conference Premier, after beating A.F.C. Telford United in the 2009 Conference North play-off Final, with Turnbull finishing the season with 49 appearances and 2 goals. He agreed a new one-year contract with the club in May 2012 to cover the 2012–13 season. Turnbull made his 300th appearance for Gateshead on 21 December 2013 in a 4–2 defeat away to Luton Town. He left Gateshead by mutual consent on 29 June 2015 to become a physical education teacher at a school in Sunderland.
Turnbull moved into part-time football when signing for newly promoted Northern Premier League Premier Division club Darlington on 29 June 2015. He played 35 of Darlington's 46 league matches in his first season as Darlington finished as champions and were promoted to the National League North. He was again a regular as Darlington finished fifth in the National League North in 2016–17, scoring his first Darlington goal in a 2–0 home win over Gloucester City on 14 January 2017. Although finishing in a play-off position, Darlington were denied entry to the play-offs due to their Blackwell Meadows ground not meeting seating requirements. On 5 October 2017, Turnbull and Gary Brown were appointed joint caretaker managers of Darlington. They remained in charge until Tommy Wright was appointed manager, achieving their only victory in their third and final match in charge on 21 October 2017 against Bradford Park Avenue.
South Shields
On 25 May 2018, Turnbull left Darlington to join his hometown team South Shields.
Turnbull plays as a central midfielder and is comfortable playing with either foot. He has been described as fiercely competitive and "is more than capable of getting forward to support the front men and chipping in with his fair share of goals".
Career statistics
Managerial statistics
Honours
Gateshead
Northern Premier League Premier Division play-offs: 2007–08
