Billy Johnson from Norwich City's Shadow Squad, managed by Adam Drury, was called up to Norfolk's Under-18 representative side on 27th September 2016. He also played for Beccles Town.

On 6th August 2018, Billy Johnson joined Norwich City on a one year deal with the intention he will play for both Norwich's Under-18s and Under-23s. He had spent time training with the Canaries in the 2018 pre-season and has subsequently been offered a deal. He had previously trialled with Peterborough

On 15th January 2019, he signed for Lowestoft Town on loan.

Johnson signed a new one year deal with City on 20th February 2019.

On 14th January 2020 he was loaned to Braintree,of the National League South, until the end of the 2019/20 season. He was a regular for them until the season ended early due to the Coronavirus outbreak.

On 27th May 2020 it was reported that Johnson would be leaving Norwich. On 22nd July 2020 it was announced that he had signed for Stevenage based upon his brilliant performances for Braintree according to manager Alex Revell. He made his professional debut in the club's 3–2 defeat to Milton Keynes Dons  in the EFL Trophy on 6th October 2020

He was released by Stevenage on 15th May 2021.

He then signed for Braintree from where he kept 8 clean sheets in 11 games in the 2021/22 season.

At the end of December 2022, Billy signed for Gloucester City of the National League North. In March 2023 he moved back to Braintree but had dual registration with Leiston.

On 10th July 2023 he signed for Southern League Premier Division Central side Kettering Town.

He joined Leiston Town of the Southern League in February 2024. On 21st March 2024 he joined Wroxham (Isthmian League) on dual registration forms.

He then signed again for Leiston on 11th June 2024 after long standing custodian Sam Donkin departed the club last week.