[3] Standing at five foot and four inches and weighing less than 10 stone, Carlin became well known for his diminutive size and his tenacity on the pitch,[5][6] being described by Brian Clough as "a belligerent, aggressive little Scouser".
[9] The following season, Carlin was handed his professional debut for the club in the Second Division at the age of 19, playing in a 2–2 draw with Brighton & Hove Albion on 10 October 1959.
[5] At Halifax, Carlin was relegated for the first time in his career during his first season at the club, dropping into the Fourth Division,[9] but made his name as a midfield schemer before joining Carlisle United in 1964 for a fee of £10,000.
[5] He helped Carlisle win promotion to the Second Division, despite suffering a broken leg during the 1965–66 season,[6] before joining Sheffield United in 1967 for £40,000, signing a five-year contract with the Blades, where he spent one season before attracting the interest of Derby County, whose manager Brian Clough had followed his progress for several years and had previously attempted to buy Carlin during his time with Carlisle but had been charged by the Football Association after making an illegal approach to him.
In April 2011, Carlin's house was targeted by burglars who broke into his home using a spade and stole his winners medal from the 1968–69 season with Derby County and the gold commemorative bracelet that was awarded to each player.