Craig Bellamy's side enjoyed a comprehensive 4-1 win over Iceland in Cardiff as they signed off the November international break in style.
The result means Wales finished top of Group 4 in League B on 12 points thanks to three wins and three draws as they achieved promotion to League A, where the continent's biggest and best await.
Turkey, who lost 3-1 to Montenegro, finished second and will now contest a play-off with a third-placed League A team to decide their fate for the next edition of the tournament.
It was Montenegro's first and only win of the latest Nations League campaign.
Wales last competed in League A during the 2022/23 season, so it will not be unfamiliar territory.
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But it highlights the strong work of Bellamy, who took over from predecessor Rob Page in July and remains unbeaten from his six games in charge.
It was Iceland who drew first blood through Andri Gudjohnsen, the son of former Chelsea and Barcelona marksman Eidur.
Gudjohnsen's seventh-minute strike marked the first time Wales had gone behind under Bellamy's tenure.
However, Wales slowly worked their way back into the contest and found their equaliser in the 32nd minute via Swansea City forward Cullen.
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Cullen then fired Wales into the lead with his second of the evening during first-half stoppage time.
The 25-year-old's goals were his first and second at international level on his seventh appearance for the national team.
His two-goal haul was also the first time a Welsh player had their first two international goals come in the same game since now talkSPORT pundit Dean Saunders accomplished the feat against Canada in May 1986.
It was then Cullen's turn to become the provider for Wales' third when he played Brennan Johnson through on goal, and the Tottenham forward made no mistake with the finish.
Fulham man Harry Wilson added the icing on the cake in the 79th minute with a thunderous 20-yard strike.
It was Wilson's fourth goal for Wales throughout the latest Nations League campaign and his 12th in total.
With promotion secured, Bellamy and his players can now fully focus on qualifying for the 2026 World Cup, which kicks off next March.