Finlay began for WWE in 2004, wrestling in a match against Jamie Noble at a house show in Glasgow, Scotland, and promos began airing for his impending in-ring return in December 2005. His gimmick was that of a proud native Irishman who loves to fight. At the age of 48 Finlay made his first televised WWE match on the January 20, 2006 airing of SmackDown! against Matt Hardy, which ended in a disqualification loss for Finlay. After the match, Finlay dragged Hardy to the steel ring steps and smashed Hardy's face into them with his boot, quickly establishing himself as a villain. This helped to give him the nickname, "The Fighting Irishman". Finlay continued to establish himself on the SmackDown! roster. During February and March 2006, Finlay was involved in a feud with Bobby Lashley which began when Finlay cost Lashley his unbeaten streak by interfering in Lashley's match with JBL at No Way Out. This feud would see the pair brawl on many occasions, including a parking lot segment in which Lashley tried to overturn a car onto Finlay. Later, the pair competed in a Money in the Bank qualifier Lumberjack match that Finlay won. During this time, Finlay began to wield a shillelagh as a weapon. On April 2, Finlay competed at his first WrestleMania, WrestleMania 22. He faced five other WWE Superstars, from both the Raw and SmackDown! brands in a Money in the Bank ladder match which also included Lashley, who won a last chance battle royal. This match was eventually won by Rob Van Dam. Finlay next entered the King of the Ring tournament on SmackDown!, defeating his first round opponent Chris Benoit before being beaten by his rival Lashley, who advanced to the finals at Judgment Day. Finlay helped the other finalist, Booker T, defeat Lashley in the King of the Ring finals. At the same pay-per-view, Finlay lost to Chris Benoit.
Beginning on the May 26 episode of SmackDown!, Finlay was joined by Hornswoggle, who came out from under the ring to attack Finlay's opponents. Finlay then joined forces with William Regal as loyal subjects of the newly renamed King Booker and his Court. Both men were later "knighted" by the King, and Finlay briefly used the name Sir Finlay. During his time as part of the court, Finlay picked up a win against World Heavyweight Champion Rey Mysterio in a non-title match before defeating the court's main foe Bobby Lashley to take Lashley's United States Championship. Finlay defended the championship on several occasions, often with help from Hornswoggle, and even defended it against fellow court member William Regal at The Great American Bash. He lost the title to Mr. Kennedy on the September 1 edition of SmackDown! in a triple threat match that also involved Bobby Lashley. After the title loss, Finlay continued to attack and wrestle threats to court leader Booker and his newly won World Heavyweight Championship, including Lashley and Batista. Finlay defeated Booker in a non-title singles match and lost in a four way match at No Mercy for the title.
After leaving the court, Finlay continued to feud with Batista. On the November 10 edition of SmackDown!, he returned to fight Batista in a singles match which he lost after Batista pinned him following a spinebuster. At Armageddon, Finlay and King Booker faced Batista and his partner, the WWE Champion John Cena from Raw, but were defeated.
Shortly after the Royal Rumble, Finlay began a feud with The Boogeyman. At No Way Out, he and Little hornswoggle defeated The Boogeyman and The Little Boogeyman. Finlay earned a place in the Money in the Bank ladder match at WrestleMania 23; however, Mr. Kennedy won the match. Following this loss, he would feud with Kennedy, after an attack on Hornswoggle during the Money in the Bank match, and Jamie Noble for assaulting Hornswoggle after he won the Cruiserweight Championship. Finlay's feud, however, would quickly redirect to Kane after Finlay spilled coffee on him. The two also became involved with Batista and The Great Khali. At Saturday Night's Main Event, he and Khali lost to Batista and Kane. Kane beat him at SummerSlam. Finlay defeated Kane in a Belfast Brawl rematch a few weeks later.
Finlay and Hornswoggle briefly separated when Hornswoggle was stripped of the Cruiserweight Title, and briefly moving to Raw with his "illegitimate father". Finlay then feuded with Rey Mysterio by attacking him during a confrontational interview with JBL. At No Mercy, Finlay faked an injury after taking a bump to the outside. Once placed on a stretcher, Finlay suddenly rose and attacked Mysterio, forcing Rey to be carried out on a stretcher. The feud continued at Cyber Sunday, where fans voted for the two to face-off in a Stretcher match. Mysterio came out on top, but Finlay retaliated with a victory on the November 9 edition of SmackDown! The feud intensified further when the two faced on opposite teams at Survivor Series.
Finlay reunited with Hornswoggle by coming to rescue him in a match against The Great Khali, effectively becoming a fan favorite. At Armageddon, Finlay was placed in a match with Khali. Finlay scored an upset win after Hornswoggle interfered by hitting Khali in the groin with a shillelagh. Still battling Khali and his translator, Ranjin Singh, with Hornswoggle, he qualified for the Royal Rumble. He drew number 27, but was immediately disqualified when he entered before his time to save Hornswoggle. Hornswoggle, who was also a competitor in the Rumble, was disqualified as well. Finlay fought in the Elimination Chamber match at No Way Out, ultimately being pinned after a chokeslam on the steel floor by The Undertaker.
Finlay then began making occasional appearances on Raw to protect Hornswoggle from Mr. McMahon, who had been showing him "tough love". After Hornswoggle was injured in a Steel Cage match by JBL, Layfield revealed that Finlay was Hornswoggle's father, and not McMahon. A week after, on March 3, Finlay admitted to this fact. At WrestleMania XXIV he was defeated by Layfield in a Belfast Brawl, where Hornswoggle also reappeared.
As part of the 2008 WWE Supplemental Draft, Finlay was drafted to the ECW brand. He and Hornswoggle challenged The Miz and John Morrison for the WWE Tag Team Championship at Night of Champions, but lost. Throughout the rest of 2008, Finlay challenged for the ECW Championship. He, along with Matt Hardy, Chavo Guerrero, The Miz and ECW Champion Mark Henry participated in the first ever Championship Scramble match at Unforgiven which Matt Hardy won. He defeated Mark Henry on an episode of ECW to earn an opportunity for the ECW championship, but was defeated by the champion Matt Hardy. Then he started a feud with Henry, and the feud ended when Finlay defeated Henry in a Belfast Brawl match at Armageddon. Finlay became the first person to defeat ECW Champion Jack Swagger on the February 3, 2009, edition of ECW, and, as a result, became the number one contender and challenged Swagger at No Way Out, but was unsuccessful. In the 2009 Supplemental Draft, Finlay and Hornswoggle were separated when Hornswoggle was drafted to Raw. In late May 2009, Finlay suffered a legitimate eye injury, sidelining him temporarily. He returned on June 16, attacking ECW Champion Tommy Dreamer, Christian, and Jack Swagger. At The Bash, he participated in a Championship Scramble match for the ECW Championship, which also included Christian, Jack Swagger and Mark Henry, however the defending champion Tommy Dreamer retained. Finlay did not return to ECW after that.
He was traded to the SmackDown brand on June 29, 2009. His first match returning to the brand was against Ricky Ortiz, which Finlay won. He went on to form part of John Morrison's team at Survivor Series, but he was eliminated by Sheamus. In late 2009, he began pursuing the Intercontinental Championship by feuding with Drew McIntyre. In 2010, Finlay became a full-time trainer and agent.
Finlay was released in March 2011.
Finlay returned on July 11, 2012 as a backstage producer.