Craig Bellamy’s side completed an unbeaten six-game campaign and overhauled Turkey – 3-1 losers in Montenegro – at the top of the group to secure elevation to League A.
That almost certainly handed Wales a World Cup play-off place to fall back on should they not automatically qualify for the 2026 tournament.
Promotion was achieved the hard way following Andri Gudjohnsen’s early strike, and for 25 minutes Wales were staring at a relegation play-off against a League C runner-up.
Swansea striker Cullen, winning his seventh cap, turned the tide with a first-half double before Brennan Johnson and Harry Wilson crowned victory with wonderful strikes.
It was Wilson’s fourth goal in this Nations League campaign and his 12th in total for Wales.
Iceland must have felt at home in chilly Cardiff with snow showers during the day and temperatures just above freezing.
Bellamy had not been afraid to make changes for the second game of these double-headers, making five alterations in September and seven in October.
On this occasion, following Saturday’s creditable 0-0 draw in Turkey, Bellamy made four changes with Karl Darlow, Connor Roberts, Jordan James and Sorba Thomas making way.
In came Danny Ward, Ben Cabango, Cullen and Daniel James as Mark Harris, forced off at half-time in Kayseri following a nasty clash of heads with team-mate Joe Rodon, retained his place in the line-up. Rodon also started to win his 50th cap.
Iceland made a fast start with Ward tipping over Isak Bergmann Johannesson’s drive and were ahead after seven minutes.
Orri Oskarsson stole behind Cabango to meet Johann Berg Gudmundsson’s cross and force a fine save from Ward.
But Gudjohnsen was on the rebound in a flash and squeezed the ball through the legs of Ward with a finish his father Eidur, the former Chelsea and Barcelona forward, would have admired.
It was the first time Wales had fallen behind under Bellamy, and they were slow to respond as Iceland defended comfortably and sprang lively attacks.
Cullen sent a grasscutter at Hakon Valdimarsson and drew Wales level after 32 minutes by nodding home Johnson’s cross.
Wales survived as Cabango headed away Jon Dagur Thorsteinsson’s deflected cross from under his own crossbar and Gudjohnsen flashed over.
But Wales finished the half strongly as Valdimarsson denied Johnson and Harris before Wilson won possession on the stroke of half-time with a sliding challenge.
Harris slipped James through and his saveable shot was pushed into Cullen’s path, who nudged the ball past Sverrir Ingi Ingason on the goal line from three yards.
Valdimarsson saved from Ben Davies and Wilson, but Iceland continued to threaten Ward’s goal and Welsh uneasiness manifested itself in Bellamy being booked for complaining to the fourth official.
Ward reacted well to help Mikael Egill Ellertsson’s shot over and the unmarked Thorsteinsson lacked composure when the ball fell at his feet in the box.
Wales were able to breathe more easily after 65 minutes after Cullen added an assist to his brace, sending Johnson scampering away to bury his fifth international goal low past Valdimarsson.
Cullen was also involved in the fourth after 79 minutes, battling for possession inside the Iceland half.
Wilson did the rest with an unerring 20-yard shot to confirm that Wales will now be beginning their World Cup campaign rather than entering the Nations League play-offs.
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