On 27 August 2008, Mihaylov made his debut for FC Twente in their 4–0 defeat to Arsenal at the Emirates Stadium in the second leg of the third qualifying round for the UEFA Champions League and went on to make his Eredivisie debut in a 1–1 draw at Roda on 30 August 2008.
On 14 March 2009, Mihaylov made his second appearance for Twente in the Dutch league against Willem II, playing the full 90 minutes and earning himself a booking, managing to keep a clean sheet in Twente's 2–0 away win.
On 29 July 2009, Mihaylov came on as a substitute for midfielder Kenneth Perez, following the dismissal of starting keeper Sander Boschker in the 27th minute of Twente's away game against Sporting CP in the first leg of the UEFA Champions League third qualifying round.
He stopped the penalty taken by João Moutinho and made a number of key saves to keep the scores level at 0–0.
Mihaylov played the full 90 minutes in the return leg and was not at fault for the Portuguese side's last-minute equalizing goal, which allowed Sporting CP to advance on the away goals rule.

FC Twente had become impressed with Mihaylov, not only his talent, but also his progress convinced them.
And after several years of being loaned out to Twente, on 5 February 2010 he officially was transferred for a fee of €1.8 million from Liverpool to Twente.
He signed a contract for three years with the Dutch club.

In the 2010–11 season, he established himself as the starting goalkeeper for Twente.
Having played in the Champions League, Mihaylov impressed with great appearances in the competition that season, among others by stopping a penalty-shot.
In the Champions League FC Twente drew to play against his "nightmare" club Werder Bremen, as it were mentioned by the Bulgarian media.
He only conceded one goal in the two matches played against Werder.
With further appearances against Tottenham Hotspur and the previous year's Champions League winner Inter Milan in the group, Twente managed on their debut in the Champions League nevertheless to end at third place, allowing continuation in the UEFA Europa League.
Twente was defeated in the quarterfinals by Villarreal, after having defeated Rubin Kazan and Zenit Saint Petersburg.

Mihaylov was quoted saying that Liverpool now had retained an option to buy him back from Twente, as he had played enough games for his native country's national team to qualify for a work permit.

On 23 December 2011, Mihaylov attended a ceremony in Sofia, where he was honoured with the Bulgarian Footballer of the Year award (Stiliyan Petrov and Dimitar Berbatov came second and third respectively).
The last Bulgarian goalkeeper to have received this distinction is his father Borislav Mihailov.
On 27 August 2008, Mihaylov made his debut for FC Twente in their 4–0 defeat to Arsenal at the Emirates Stadium in the second leg of the third qualifying round for the UEFA Champions League and went on to make his Eredivisie debut in a 1–1 draw at Roda on 30 August 2008. On 14 March 2009, Mihaylov made his second appearance for Twente in the Dutch league against Willem II, playing the full 90 minutes and earning himself a booking, managing to keep a clean sheet in Twente's 2–0 away win. On 29 July 2009, Mihaylov came on as a substitute for midfielder Kenneth Perez, following the dismissal of starting keeper Sander Boschker in the 27th minute of Twente's away game against Sporting CP in the first leg of the UEFA Champions League third qualifying round. He stopped the penalty taken by João Moutinho and made a number of key saves to keep the scores level at 0–0. Mihaylov played the full 90 minutes in the return leg and was not at fault for the Portuguese side's last-minute equalizing goal, which allowed Sporting CP to advance on the away goals rule.
FC Twente had become impressed with Mihaylov, not only his talent, but also his progress convinced them. And after several years of being loaned out to Twente, on 5 February 2010 he officially was transferred for a fee of €1.8 million from Liverpool to Twente. He signed a contract for three years with the Dutch club.
In the 2010–11 season, he established himself as the starting goalkeeper for Twente. Having played in the Champions League, Mihaylov impressed with great appearances in the competition that season, among others by stopping a penalty-shot. In the Champions League FC Twente drew to play against his "nightmare" club Werder Bremen, as it were mentioned by the Bulgarian media. He only conceded one goal in the two matches played against Werder. With further appearances against Tottenham Hotspur and the previous year's Champions League winner Inter Milan in the group, Twente managed on their debut in the Champions League nevertheless to end at third place, allowing continuation in the UEFA Europa League. Twente was defeated in the quarterfinals by Villarreal, after having defeated Rubin Kazan and Zenit Saint Petersburg.
Mihaylov was quoted saying that Liverpool now had retained an option to buy him back from Twente, as he had played enough games for his native country's national team to qualify for a work permit.
On 23 December 2011, Mihaylov attended a ceremony in Sofia, where he was honoured with the Bulgarian Footballer of the Year award (Stiliyan Petrov and Dimitar Berbatov came second and third respectively). The last Bulgarian goalkeeper to have received this distinction is his father Borislav Mihailov.
Nikolay Mihaylov