Harbison studied at St Malachy's College in Belfast. He became active in the Irish Parliamentary Party, acting from 1906 until 1910 as the election agent for William Redmond and Tom Kettle. In 1911, he was elected to Tyrone County Council.
After attending the Irish Convention, he was elected to Westminster at the 1918 East Tyrone by-election, after Redmond resigned it to contest Waterford City. At the 1918 general election, Harbison was elected in North East Tyrone.
At the 1921 Northern Ireland general election, Harbison was elected on an abstentionist platform in Fermanagh and Tyrone. In 1922, he was elected in the Westminster constituency of Fermanagh and Tyrone for the Nationalist Party. He stood down from the Westminster seat at the 1924 election, and in 1927 took his seat at Stormont. In 1929, he stood down from his Stormont seat, but was again elected to Westminster, serving until his death a year later.
- "General Registrar's Office". IrishGenealogy.ie. Retrieved 20 April 2017.
- Hansard 1803–2005: contributions in Parliament by Thomas Harbison
|Parliament of the United Kingdom|
| Member of Parliament for East Tyrone
1918 – 1918
|New constituency|| Member of Parliament for North East Tyrone
|New constituency|| Member of Parliament for Fermanagh & Tyrone
With: Cahir Healy
| Member of Parliament for Fermanagh & Tyrone
With: Joseph Devlin
|Parliament of Northern Ireland|
|New parliament|| Member of Parliament for Fermanagh and Tyrone
With: Arthur Griffith 1921–1922
William Coote 1921–1924
Seán Milroy 1921–1925
William Thomas Miller 1921–1929
James Cooper 1921–1929
Seán O'Mahony 1921–1925
Alexander Donnelly 1925–1929
Rowley Elliott 1925–1929
Cahir Healy 1925–1929
John McHugh 1925–1929
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