Born in Scotland, to James and Christina (née Naismith) Plain, where he was educated, he migrated to Australia in 1890, where he became a farm worker and gold miner at Lara, Victoria.

[1] In 1908, he was elected to the Victorian Legislative Assembly as the Labor member for Geelong.

[1] In 1917, he left the Assembly to contest the Australian Senate as a Nationalist candidate for Victoria.

He was defeated in 1922, but was re-elected in 1925; he was appointed early to the Senate after the death of Edward Russell.

Plain was buried in the Presbyterian section at the Geelong Western Cemetery with his sister Elizabeth, who had died in 1960.