Forsyth enlisted in the Queensland Mounted Rifles Regiment as a trooper in November 1885. He served as a non commissioned officer before being commissioned a second lieutenant in the Queensland Mounted Infantry on 18 July 1892. He was promoted to lieutenant on 4 November 1892 and captain on 22 September 1896. He joined the Queensland Permanent Forces at a staff lieutenant on 1 August 1897, and was promoted to captain on 1 February 1901. Forsyth served as adjutant of the 4th Queensland Mounted Infantry from 1 March 1901 to 27 February 1902, of the 2nd Queensland Mounted Infantry and 4th Infantry Regiment from 28 February 1902 to 30 June 1902, and of the 2nd Queensland Mounted Infantry again from 1 July 1902 to 12 November 1905.