Among the other businesses he started or was involved in included Cash and Company Cork, Baldoyle and Cork Race Park Meetings, the City of Cork Steamship Company, Passage Docks Shipbuilding Company, the Bristol Steam Navigation Company and Arnotts Brewery Cork. He acquired the Irish Times and The Northern Whig newspapers, though he later disposed of the Whig over disputes relating to its editorial policy. His family retained a connection with the paper until the 1960s, although they had disposed of their interest earlier.

Arnott was elected Lord Mayor of Cork three times, in 1859, 1860 and 1861. He was Justice of the Peace for Cork City and County and served as Member of Parliament for Kinsale between 1859 and 1863. Arnott was created a Knight Bachelor by the Lord Lieutenant of Ireland in 1859 and became a Baronet, of Baily, in the County of Dublin on 12 February 1896.

He was a philanthropist and was heavily involved into an investigation in the treatment of children at the Cork workhouse. In this period the Irish Poor Law Relief Bill was going through Parliament and he sat on the select committee. There is a plaque on St Patrick's Bridge in Cork that commemorates its opening by Arnott on 12 December 1861.

He married Mary, the daughter of John James McKinlay.