In 1837, at the early age of 22, Dillwyn became a Glamorganshire magistrate and in 1843 played a prominent role alongside his brother,
John Dillwyn Llewelyn (who had taken his mother's maiden name after inheriting the Penllegare estate) in preventing the
Rebecca riots from engulfing Glamorgan as they had neighbouring
Carmarthenshire. In 1855 he was elected Member of Parliament for
Swansea District, succeeding J.H. Vivian who had held the seat since 1832. He held the seat and its central successor for 37 years and in all but the last years with few challenges (e.g. opposed in 1874 by Charles Bath of Ffynone, comfortably defeated). In Parliament, Dillwyn had built a reputation by the 1860s as an advanced radical and, at least until the election of men such as
Henry Richard, he was regarded as the leader of the Welsh Liberal Party from his regular corner seat below the gangway. Not an effective orator – one obituary even alluded to his 'remarkable inability to make a coherent speech'. – he took praise as a critic of privileges of his own church. It was significant he was an Anglican and this made his support for the campaigns of the
Liberation Society against the status of that Established church more effective. He introduced bills in 1860 and 1863 to enable dissenters to be elected as trustees of endowed schools and his motion on the Church of Ireland (28 March 1865) influenced Gladstone's gradual move towards disestablishment. From 1870 he first supported the disestablishment of the Welsh Church (Anglican Church in Wales), in 1873 moved an anti-clerical amendment to the Endowed Schools Act and from 1883 he annually moved for such disestablishment. He favoured Local Option to enable Councils to close of all public houses within a given area. During this period Dillwyn came to be regarded as a conspicuous Radical and was an active supporter of the
Reform Act 1867. During the passage of the Second Reform Bill Dillwyn's involvement as a leading member of the 'Tea Room' cabal of disaffected Liberals in April 1867 helped to bring about household suffrage, a measure which led to an overnight increase in the urban electorate throughout Great Britain. At the
1865 General Election he was instrumental in promoting the candidature in
Cardiganshire of
Evan Matthew Richards, a fellow Swansea industrialist. This election was notable for the allegations of clerical influence and intimidation. In Parliament, Dillwyn championed the cause of Cardiganshire farmers who were evicted for their votes in 1868 election. Similarly, in the 1880s, he supported the Denbighshire tenantry who agitated against tithes. In 1887 he and
Stuart Rendel affirmed the Welsh Liberal Party's support of
Irish Home Rule. After the
Redistribution of Seats Act 1885, the third Reform Act, Dillwyn became MP for the new seat
Swansea Town from 1885 to 1892. Dillwyn was opposed in 1885 by little known 22-year-old Tory, W.H. Meredyth who belonged to a leading Anglo-Irish family and drew much of his support during the campaign from aristocrats who came to Swansea to support him. Dillwyn's slashed majority demonstrated that the now marginal political status locally, heightened by the Swansea Town seat excluding most of the overwhelmingly working-class suburbs and contributory boroughs. In 1886, he increased his majority. == Scientific interests ==