At a young age, Allen considered a career in football, having trials with Sheffield Wednesday and Nottingham Forest before concentrating on snooker.

[8] As a local player, he was invited and made an immediate impact, defeating Steve Davis and John Higgins to reach the quarter-finals, before losing to Stephen Hendry.

[8] In his first year on the tour, he reached the last 32 of the 2005 UK Championship and the 2006 Welsh Open, losing 2–5 to the then World Champion Shaun Murphy after leading 2–0.

He faced Ronnie O'Sullivan in the second round; confident and unaffected by his opponent's reputation, Allen beat the defending champion 13–11 for a quarter-final place in the 2009 World Championship.

[13] Soon after his run in the World Championship, Allen won his first professional tournament, the 2009 Jiangsu Classic beating home favourite Ding Junhui 6–0 in the final.

[15] Five days later, Allen made the first 146 break in World Championship history, and the second of his career after defeating Mark Davis 13–5 in the second round.

[18] The next ranking event was the Shanghai Masters where Allen reached the second round and held a 4–2 advantage over Shaun Murphy before losing the next 3 frames and being edged out of the match 5–4.

[20] Allen made it to his first ranking event final at the 2011 UK Championship by beating Adrian Gunnell, Ali Carter, Marco Fu and Ricky Walden.

He beat qualifier Jimmy Robertson in the first round, before exacting revenge over Judd Trump for his defeat in York, by coming back from 0–3 down to triumph 5–4.

[30] Allen began the season with second round losses to Mark Williams at the Wuxi Classic and Judd Trump in the Shanghai Masters.

[34] At the Masters Allen came past Mark Davis 6–2, but was then narrowly beaten 5–6 by Neil Robertson in a high-quality encounter in the quarter-finals.

[31] Allen comfortably won every match he played at the event as he beat Ryan Day 5–2, Robert Milkins 5–2, Ricky Walden 5–1, John Higgins 6–2 and Matthew Stevens 10–4 in the final to capture his second ranking title.

[36] Allen's Antwerp Open win from earlier in the season helped him finish eighth on the Players Tour Championship Order of Merit to qualify for the Finals where he lost 3–4 in the quarter-finals to Ding, who made a 147 and two further centuries during the match.

[40][41] He also won the next European Tour event, the Kay Suzanne Memorial Cup by battling past former world champions Neil Robertson and Graeme Dott in the quarter-finals and semis respectively.

[44] Allen came close to recording a hat-trick of World Open titles as he won through to the semi-finals, but was beaten 6–4 by Shaun Murphy.

[45] His two titles earlier in the season meant Allen was the number one seed for the PTC Finals where he lost in the quarter-finals 4–2 against Gerard Greene.

[55] The rest of the season would prove to be disappointing for Allen as he couldn't advance beyond the last 16 of any event and after he lost the last five frames against Barry Hawkins in the second round of the World Championship to be beaten 13–11, he stated that the match had summed up his year as he was great in patches but overall came up short.

[58] Allen swept to the final of the Bulgarian Open by whitewashing Mark Williams 4–0 and did not drop a frame against Ryan Day either as he took home the title, his first in 15 months.

However, Allen succeeded in the Masters beating Luca Brecel, Ronnie O'Sullivan, John Higgins and runner-up Kyren Wilson 10-7.

Despite this, Allen reached UK Championship final after a won decider in semi-final against Stuart Bingham at the start of December.

Just after UK championship Allen added his 5th ranking victory by winning last tournament in year 2018 Scottish open by defeating Shaun Murphy in the final.

[87] In December 2024, Allen won the non-ranking Riyadh Season Snooker Championship with a 5‍–‍1 victory in the final over Luca Brecel.

Bingham responded by calling Allen an idiot and the tension between the two was clearly high as they were drawn to play each other in the second round of the 2011 Australian Goldfields Open.

[90] Bingham fulfilled his pre-match words by defeating Allen 5–3 and also went on to win the tournament by coming back from 8–5 down to beat Mark Williams 9–8 in the final.

[97] In a post-match press conference after his first round win in the 2011 UK Championship, Allen called for the chairman of the World Professional Billiards and Snooker Association (WPBSA), Barry Hearn, to resign.

[100] The WPBSA announced that Allen would be facing a disciplinary committee for swearing when talking about Hearn and that he could be charged with bringing the game into disrepute.

[102] Upon learning that Hearn had been on holiday for the entirety of the event, Allen said it was a "joke" and only confirmed the views which he had expressed earlier in the week.

[105] Just two months later while playing in the World Open in Haikou, China, Allen described the conditions on his Twitter page as "horrendous", following up with: "Journey a nightmare.

Following his defeat at the 2012 World Snooker Championship to Chinese qualifier Cao Yupeng, he openly accused his opponent of cheating and unsporting behaviour for not owning up to what he perceived to be a foul.

[119][132] During the COVID-19 pandemic, Allen spent three to four hours a day delivering groceries and prescription medications to vulnerable people in Antrim.