Six-foot Sid makes the switch to Hampshire, after spending the past season with Woking in The Vanarama National League.
Starting his football career with Millwall, Nelson signed for The Lions at the age of eight and went on to make his senior debut a decade later.
The Lewisham born defender then experienced five loan moves – starting with Newport County where helped ensure their survival in League Two – before spells with Yeovil Town, Chesterfield, Swindon Town and Tranmere Rovers.
During that loan with the Super White Army, Sid became a Wembley winner, beating Newport County 1-0 in the Sky Bet League Two Play-Off Final in May 2019. He made his move to Prenton Park permanent a month later, signing a two-year contact.
Nelson joined Northampton on a one-year deal in July 2021, although was forced to miss the majority of the season due to injury.
The no-nonsense centre-back then signed for Darren Sarrl’s Woking last July, spending time on loan with Welling United during 2022/23 too.
Manager Spencer Day said: “What a signing! Sid has roughly 100 Football League appearances, including in the Championship and is only 27.
“Can play any of the centre half roles. He suffered a horrible injury two years ago but is now fully fit and raring to get going. I’m hoping he can follow the path of Kev Lokko by us using each for mutual gain.
“Another player that had multiple offers but choose us and has the most infectious character that will become a fans favourite overnight!”
Sid’s first words as a Farnborough player: “It’s an absolute pleasure to be here. For me, it’s a club on the up. The project that I know is going on down here is something I want to be involved in.
“I had good reviews from a few people. I spoke to the Manager and it was a no brainer in the end. As a club, we’ve got to be aiming for the top end of the table and trying to kick on for next year.”