His father owned much of land in Waimānalo on the east coast of the island of Oʻahu, starting a horse and cattle ranch in the 1840s.

He traded racehorses with Leland Stanford and Pierre Lorillard IV, and operated a railroad and a steamship to the estate.

The kingdom faced a series of political crises, including a need for an election for monarch after Kamehameha V and Lunalilo both died without naming heirs.

Although many ancient Hawaiian customs had faded (due to influence of conservative Christian missionaries, for example), Cummins staged great revivals of ceremonies such as traditional hula performance.

[7]: 307–309 In the legislature he advocated for the Reciprocity Treaty of 1875 with the United States, which helped increase profits in the sugar industry, and his fortunes grew.

He left the sugar business to William G. Irwin, agent of Claus Spreckels, and developed a commercial building called the Cummins Block at Fort and Merchant streets in Downtown Honolulu.

[10] After the overthrow of the Kingdom of Hawaii in early 1893, Liliʻuokalani asked Cummins to visit the US to lobby for its help in restoration of the monarchy.

[12][13] However, on the voyage to the west coast, William T. Seward, a former Major in the American Civil War who worked for Cummins and lived in one of his homes, smuggled guns and ammunition for the failed 1895 counter-revolution.

He was sentenced to prison, but released after paying a fine and agreeing to testify against the ones actively involved in the arms trading.

[19] After the last child died in 1937, a US federal court case awarded Mamo Clark a share in the still considerable estate.