After rejection from Watford's youth team as a 16-year-old, Ifill returned to his home town, Brighton. He enrolled on a physical education course at college and played non-League football in the same Saltdean United team as his father.
Jeff Burnidge and Mick Beard scouted Ifill following a tip from a former Millwall player, and Millwall signed him as a centre-forward.
After he suffered injury at a crucial time between contracts, manager Billy Bonds gave him just one month to prove himself. Ifill responded by scoring four goals the next Saturday and two more the following week.
Injuries led to Millwall switching him to midfield, but a recurring back-injury prevented him from sprinting, a basic requirement for a winger. Despite seeing a number of different specialists, he was out for three months, and seemed in danger of having to end his career. Fortunately, a Harley Street consultant diagnosed a pelvic flaw pressing on a nerve: the day after treatment he was able to sprint again. He was a member of the team that reached the 2004 FA Cup Final, where they lost to Manchester United 3–0.
He signed for Sheffield United in May 2005 for £800,000, after making over 200 league appearances at The New Den.
In his first season at Bramall Lane, United raced into a large lead, and despite falling behind eventual champions Reading, the Blades finished as runners-up, securing promotion to the Premier League in the process.
In the top flight, Ifill fell out of favour, and boss Neil Warnock placed him on the transfer list. During the January transfer window, Ifill left the Blades and after much competition from other clubs was unveiled by Crystal Palace, having signed for a reported £800,000 fee.
At Palace Ifill's career stuttered, with him struggling for fitness. Warnock's appointment to the managers' post at Palace did not leave him without any chances, but nevertheless Ifill struggled to remain fit.
On 18 August 2008 Ifill and teammate Carl Fletcher were put on the transfer list by manager Neil Warnock. He was released by Crystal Palace on 6 May 2009 with eight other players.
On 21 July 2009, Ifill signed a two-year contract with New Zealand-based Wellington Phoenix of the Australian A-League. He scored his first goal for his new club in his second appearance netting a header off a pinpoint cross from Leo Bertos to help secure a 2–1 win over Perth Glory at Westpac Stadium. Ifill's second goal came against Adelaide United.
On 6 September 2009 two days after his goal against Adelaide, Ifill signed a contract extension to keep him at the Phoenix until the end of the 2011–12 season, with an option for an extra year.
Ifill's third goal came on 25 October 2009 as the Phoenix hammered Gold Coast United 6–0, a regular-season record for the largest win. Ifill not only scored, but provided three assists in a masterful performance.
Ifill then scored in three consecutive games for the Phoenix, resulting in two wins and a draw for the fledgling club. On 31 December 2009, Ifill scored both goals in a 2–0 win over the Central Coast Mariners, pushing the Phoenix back up to seventh position in a congested ladder.
In Ifill's first two games of 2010, Ifill provided the Phoenix with an assist to Eugene Dadi in a match against the Brisbane Roar as well as a goal against North Queensland Fury.
Ifill's tenth goal for the Wellington Phoenix came on 7 February 2010 via a penalty against Gold Coast United in a 1–0 win. The goal put the Wellington club into the top four leading into the final round of the regular season.
Ifill then scored twice more against Central Coast Mariners in the final round of the regular season to help his team to a 3–0 win, securing fourth position. In the process, Ifill joined former Phoenix marksman Shane Smeltz as record holder of most goals scored in a regular season (12).
Ifill scored in the penalty shoot-out as the Phoenix prevailed over Perth Glory 1–1 (4–2 on penalties) in the first week of the playoffs. Ifill scored an extra-time goal as the Phoenix moved past Newcastle Jets 3–1 in the second week. However Ifill had a comparatively quiet match as the Phoenix were downed 4–2 by eventual champions Sydney FC in the Preliminary Final.
Ifill was injured early in the Phoenix's game against Newcastle Jets on 18 December 2010, and it was initially feared he had broken his leg. However, later scans showed that there was no fracture, but an ankle sprain with some ligament damage. The expectation was that he would miss six weeks of the 2010/11 season.
On 8 December 2011, it was announced that Ifill had signed a two-year contract extension with the club.
In 2015 Ifill took on a player/ coach role at Wairarapa United in the Men's Central League. He runs this alongside his two Paul Ifill Football Academies in Nelson and Masterton.
Ifill plays internationally for Barbados due to parental nationality and played in two World Cup qualifying games for Barbados in June 2004 against St Kitts & Nevis.
He was recalled to the squad for three Caribbean Nations Cup 2007 Group B fixtures against St Kitts and Nevis, Antigua & Barbuda and Anguilla at the Antigua Recreation Ground. On 24 September 2006, he scored a hat-trick for Barbados during their 7–1 Gold Cup qualifying victory over Anguilla at the Antigua Recreation Ground.
Ifill was recalled for Barbados' semifinal Group G, in which he scored twice in three games, as the hosts advanced to the finals, in Trinidad and Tobago, in January 2007.
He scored all his international goals for Barbados in the 2006–07 Caribbean Nations Cup, the qualification tournament for the 2007 CONCACAF Gold Cup.
Ifill captained his country against USA in two 2010 World Cup qualification matches, which Barbados lost 9–0 on aggregate.