Between 2002 and 2015 he made 148 league appearances for a number of clubs, notably Middlesbrough, Chelsea, Crewe Alexandra and Doncaster Rovers.

He played as a midfielder for his hometown team Newton Aycliffe Youth Centre AFC but was turned into a goalkeeper by coach Arthur Vickerstaff.

In July 2007, Turnbull signed a season-long loan deal at Football League Championship side Cardiff City.

However, due to some errors in his opening games he was replaced for the 2–2 draw with Preston North End by Michael Oakes and spent the remainder of his loan spell on the bench before he was recalled by Middlesbrough on 5 October 2007 following an injury to second choice keeper Brad Jones.

[7] After being recalled, Turnbull played in two of Middlesbrough's games in the absence of first choice goalkeeper Mark Schwarzer who had broken his thumb in training.

[8] Middlesbrough manager Gareth Southgate decided not to buy a new goalkeeper and allow Turnbull and Jones to fight it out for the number one position.

[10] In June 2009 Turnbull rejected a contract offer from Middlesbrough, and informed the club that he would leave when it expired at the end of the month.

[14] He made his first team debut against Bolton Wanderers in the League Cup, coming on for the injured Henrique Hilário after 23 minutes, a game which Chelsea won 4–0.

[16] He made his Premier League debut against West Ham United in a 4–1 win as Petr Čech and Henrique Hilario respectively were both injured.

On 23 November 2010, Turnbull made another appearance in the Champions League Matchday 5 group stage against Slovakian side MŠK Žilina as Chelsea won 2–1 when they came from a goal down to beat them.

[23] Turnbull went on to make a string of impressive performances for Doncaster in 2013, which included saving a penalty in a South Yorkshire derby against Barnsley which finished 0–0 on 9 November.

At the beginning of February 2015 it was reported that Turnbull was suffering from injuries in both of his calves and a specialist had recommended a three to four week rest period.

[28] Turnbull conceded 5 goals against struggling Crawley Town on 14 February 2015, a game which equaled Barnsley's seventh straight away defeat in the league.

Turnbull missed the remainder of the 2014–15 season due to a back injury which he underwent successful surgery in March 2015 to correct.

[36] By 24 September 2016, Turnbull had fallen so far out of favour that he, along with Toumani Diagouraga and Luke Murphy, were no longer listed in the Leeds United match programme as a member of the squad,[37] and all three were training with the under-23 team.