Raith Rovers Callachan came through the
youth system at
Raith Rovers. He was promoted to the first team after agreeing a full-time contract with the club as a 16-year-old in June 2010. He spent much of the
2010–11 season in the reserves. He did, however, make his first team league debut in the final match of the campaign, replacing teammate
Iain Williamson as a 60th-minute substitute in a 3–0 defeat against
Partick Thistle on 7 May 2011.
Heart of Midlothian On 31 August 2017, Callachan signed for
Heart of Midlothian on a two-year deal, for an undisclosed transfer fee.
St Johnstone After one season at Hearts, Callachan signed for
St Johnstone on a two-year deal before the closing of the Summer transfer window. He scored his first goal for St. Johnstone on 29 September 2018 against his former club Hearts in a 2–1 defeat at
Tynecastle.
Loan to Dundee Callachan joined
Dundee on loan in January 2020 for the rest of the season. After just a few weeks with Dundee however, Callachan suffered a leg fracture during a game against
Partick Thistle that kept him out for the rest of the season. In May, St Johnstone announced Callachan would be one of a few players to leave the club once his contract ended at the end of the month.
Hamilton Academical On 29 June 2020,
Hamilton Academical's chairman confirmed that Callachan had signed for Accies.
Ross County Callachan signed a two-year contract with
Ross County in June 2021. After playing a key role for the
Staggies over two seasons, Callachan would be sidelined for an extended period after
rupturing his ACL in April 2023, and would miss the entire
2023–24 season as a result.
Motherwell On 19 June 2024, Callachan joined Motherwell on a one-year deal upon expiration of his contract with Ross County. Callachan made his debut for the
Steelmen on 3 August 2024, 16 months after his injury, in the league opener against his former club
Ross County. Shortly after, Motherwell manager
Stuart Kettlewell confirmed that Callachan had picked up a "bad" hamstring injury which would keep him out for the remainder of the season. At the end of the season, Motherwell confirmed that Callachan was departing the club upon the expiry of his contract.
Arbroath On 27 June 2025, Callachan joined the newly-promoted
Scottish Championship club
Arbroath on a permanent deal. On 22 July 2025, Callachan played his first competitive minutes in nearly a year in a 6–0 Arbroath home win over
Annan Athletic in the League Cup. On 9 August, Callachan scored his first goal since October 2022 in a league win away to
Queen's Park. ==Career statistics==