Ward became involved in local politics very quickly. He was elected to the Campbelltown (Bluff) Borough Council in 1878, despite being only 21 years old — he later became Mayor. He also served on the Bluff Harbour Board, which he eventually became chairman of.
In 1887, Ward successfully stood for Parliament, winning the seat of Awarua. Politically, Ward was a supporter of politicians such as Julius Vogel and Robert Stout, leaders of the liberal wing of Parliament — Ward's support was unusual in the far south. Ward became known as a strong debater on economic matters.
In 1891, when the newly-founded Liberal Party came to power, the new Prime Minister, John Ballance, appointed Ward to the position of Postmaster General. Later, when Richard Seddon became Prime Minister after Ballance's death, Ward became Treasurer (Minister of Finance). Ward's basic political outlook was that the state existed to support and promote private enterprise, and his conduct as Treasurer reflects this.
Ward's increasing occupation with government affairs led to neglect of his own business interests, however, and Ward's personal finances began to deteriorate. In 1896, a judge declared Ward "hopelessly insolvent". This placed Ward, as Treasurer, in a politically difficult situation, and he was forced to resign his portfolios on 16 June. In 1897, he was forced to file for bankruptcy, which legally obligated him to resign his seat in Parliament. A loophole, however, meant that there was nothing to stop him simply contesting it again — he did so, and in the resulting by-election was elected with an increased majority. Ward actually gained considerable popularity as a result of his financial troubles — Ward was widely seen as a great benefactor of the Southland region, and public perceptions were that he was being persecuted by his enemies over an honest mistake.
Gradually, Ward rebuilt his businesses, and paid off his creditors. Richard Seddon, still Prime Minister, quickly reappointed Ward to Cabinet. He gradually emerged as the most prominent of Seddon's supporters, and was seen as a possible successor. As Seddon's long tenure as Prime Minister continued, some suggested that Ward should challenge Seddon for the leadership, but Ward was unwilling.
In 1906, Seddon unexpectedly died. Ward was in London at the time. It was generally agreed in the party that Ward would succeed him, although the return journey would take two months — William Hall-Jones became Prime Minister until Ward arrived. Ward was sworn in on 6 August 1906.
Sir Joseph became the Minister of the post office and railways, during which time he continued the expansion of each service as started by his predecessors. He died in 1930 and is buried in Bluff.