Biggar sympathised with Fenianism but considered reliance on physical force against Britain to be unrealistic. He joined the Irish Republican Brotherhood after his election to parliament in 1874 and accepted a seat on its Supreme Council, but 'only with a view to winning fenian support for parliamentary politics'. However, his involvement in constitutional politics did not sit well with his more radical IRB colleagues and he was expelled from its Supreme Council in 1876 according to Jackson. According to T. W. Moody he was expelled in March 1877 on the expiration of the August 1876 ultimatum of the I.R.B.'s supreme council to its members to cease involvement with the home rule movement.