In 2015, Allen was successful in Q School and earned a tour card for the 2015–16 and 2016–17 seasons by beating Alex Taubman 4–2 in his final match of Event 2. He won his first match as a professional by beating Adam Edge 5–3 in Australian Goldfields Open qualifying, but lost 5–1 to Jamie Burnett in the second round. Allen saw off Barry Pinches and Aditya Mehta both 5–1 in the qualifying rounds for the Shanghai Masters, before losing by a reversal of this scoreline to Li Hang. He made his debut at the venue stage of a ranking event in York for the UK Championship and was defeated 6–1 by Liang Wenbo. Allen then lost 15 successive matches from August 2015 until the start of the 2016–17 season, when he beat Kurt Maflin 4–3 to qualify for the Riga Masters. Allen won a match at the venue stage of a ranking event for the first time by edging past Ross Muir 4–3 in the opening round of the Scottish Open and then lost 4–3 to Mike Dunn. A 5–4 win over Robin Hull saw him qualify for the China Open, where he was beaten 5–0 by Ronnie O'Sullivan in the second round. Allen fell off the tour at the end of the season as he was placed 110th in the world rankings.