Howell became Corbyn's deputy director for strategy and comms in February, working alongside comms chief Seumas Milne.
His eventful six months with the party saw Corbyn achieve a better-than-expected result in June's General Election - something Howell reminded Twitter users of yesterday.
Some people reminding me of this tweet, for which I got some stick at time. There was all to play for..and still is. https://t.co/G7Kt4YMhcY— Steve Howell (@FromSteveHowell) August 23, 2017
Heading back to Cardiff feeling very privileged to have worked for @jeremycorbyn & looking forward to continuing to play a role part-time.— Steve Howell (@FromSteveHowell) August 23, 2017
The former BBC journalist stood down as CEO of Freshwater, the agency he founded in 1997, when he took the Labour job, but retained a non-executive director role.
Howell, who lives in Cardiff, will continue to act as a non-executive at Freshwater, and will continue to advise Corbyn on a part-time basis.
Politics Home says that suggestions Howell and Milne had fallen out were denied by a party source, adding that it was always planned for the Welshman's appointment to be for a limited period.
He told PRWeek he also planned to spend more time writing.