Later that year, he defeated Phil Taylor in the last 16 of the John McEvoy Gold Dart Classic in Killarney, eventually losing in the quarter-finals to Colin Osborne.

In April, Cullen reached his first PDC Pro Tour final at the sixth UK Open Qualifier in Barnsley, losing 6–1 to Taylor.

[8][9] In April, he earned a place in the Austrian Darts Open by defeating Sam Hill and John Bowles in the UK qualifier.

[10] He played Brendan Dolan in the first round and won 6–4,[11] and then continued his run by beating Simon Whitlock 6–4, despite his opponent hitting a nine darter.

[12] Cullen defeated Mark Walsh to make the quarter-finals, where he was this time on the wrong end of a 6–4 scoreline as he lost to eventual winner Justin Pipe.

[16] Cullen reached the last 16 of the UK Open with victories over Jim Walker (9–4) and Colin Lloyd (9–8),[17] but then let a 4–0 and 5–1 lead against Jamie Caven slip to lose 9–8.

[19] Cullen qualified for the 2013 World Championship by finishing 36th on the 2012 ProTour Order of Merit,[20] claiming the fifth of sixteen spots that were available for the highest non-qualified players.

[25] He reached the quarter-finals of this event for the second year running by surviving one match dart from Raymond van Barneveld to win 6–5, but was then beaten 6–0 by Mervyn King.

[32] Cullen won through to the same stage of the 16th event with impressive victories over Dave Chisnall, Andy Hamilton and Michael van Gerwen, but this time lost 6–3 to Brendan Dolan.

[37] Cullen's third and final quarter-final of 2015 was at the 19th Players Championship after he ousted Ronnie Baxter, Ian White, Jamie Caven and Steve West, before losing 6–4 to Stephen Bunting.

[38] He took the final qualifying spot from the Pro Tour Order of Merit for the 2016 World Championship, but lost in the opening round for the sixth year in a row as he won just two legs during a 3–0 defeat to Jelle Klaasen.

[39][40] At the UK Open Cullen beat Mark Frost 9–1, Alan Norris 9–6 and Stephen Bunting 9–7 to play in his first major quarter-final.

[48] Cullen made his debut at the World Grand Prix and he won the first set against Simon Whitlock, before losing six of the next seven legs to bow out.

[51] Cullen eased past Steve Brown 6–1 and Rowby-John Rodriguez 6–0 at the Players Championship Finals, but then averaged 71.13 as Raymond van Barneveld knocked him out 10–2 in the third round.

[52] After breaking into the top 32 on the Order of Merit during 2016, it meant Cullen was a seeded player at the World Championship for the first time in his career.

Gurney missed one match dart and Cullen won on double four and said afterwards that he had relied on his natural talent in the past, but in the last year he had also been working very hard on his game.

[62] Cullen defeated Daryl Gurney, Gary Anderson 10–1 with an average of 106.30, Michael van Gerwen and José de Sousa en route to the final.

[70] He also reached his maiden World Grand Prix semi-final after wins against Mike De Decker, Ross Smith and Chris Dobey.

[82] Cullen is a supporter of rugby league club Wigan Warriors,[83] and conducted the draw for the Challenge Cup in 2024 alongside Michael Smith.