On 27 September 1886 at St Patrick's Church in Adelaide, he married Bridget Fitzpatrick, with whom he was to have two sons.
He joined the Labor Party and won the Legislative Assembly seat of Pilbara at a by-election on 23 July 1906 following the resignation of Independent member James Isdell.
In March 1917, some Labor members of parliament, including Underwood and former premier Scaddan, to support the Nationalist Senate team for the 1917 federal election over the conscription issue.
He resigned on 26 March 1919 after he, Premier Lefroy and James Gardiner were stranded in Melbourne for several weeks due to quarantine regulations relating to the influenza epidemic.
Little is known about his life after politics, although he resided in outer suburban Perth where he maintained a small farm.