Howorth was commissioned lieutenant on 4 January 1757, and during 1772-3 served in this rank aboard Vice-Admiral Thomas Pye’s flagship at Portsmouth, the Barfleur 98, Captain Edward Vernon.
In 1776 he was serving as a lieutenant aboard the Eagle 64, Captain Henry Duncan, the flagship of the commander-in-chief in North America, Vice-Admiral Lord Howe. He was promoted commander of the bomb Carcass on 1 February 1777, and participated in the Philadelphia Campaign of August-November in command of the Swift 14. During Commodore Sir George Collier’s campaign from May to August 1779 he commanded the Galatea 20 in the Penobscot expedition, and in the same year he captured the privateers Revenge on 28 April, Hawk on 4 September, Saratoga on 11 September and Levrette on 25 October. He retained this vessel into the following year.
Having been posted captain on 4 April 1781, Howorth commanded the Victory 100, flagship of Vice-Admiral Hyde Parker and Commodore John Elliott, from 17 April to 22 November, participating in the Channel fleet operations of June – November. In 1782 he briefly flew the flag of Rear Admiral John Lockhart-Ross aboard the Bienfaisant 64 during the Channel fleet campaign of April to August and he had the same vessel as a private ship at the relief of Gibraltar on 18 October and in the subsequent action off Cape Spartel
Howorth became a superannuated admiral in 1799 and died in 1801 at his residence in Hay-on-Wye, Breconshire.