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.