The Spartan South Midland Premier Division runners-up were invited to step up by the FA after fans of London Colney voted to decline promotion to Ryman North at their Annual General Meeting.
After the FA's League Committee confirmed their late promotion this week, the Hertfordshire club will be formally admitted into the competition at the Isthmian League's AGM at the Hilton Hotel Wembley on Saturday 17 June.
The Blues have told their fans they can't wait to see their Hertingfordbury Park home hosting Ryman North football over the course of the 2017-18 season and are confident they can more than hold their own.
Hertford Town chairman Peter Sinclair said: "Everyone at the club is delighted to be back in the Ryman after a gap of 15 years and it's fully deserved following a positive campaign under Gavin Kelsey. A second placed finish would have been enough to see Hertford Town promoted in the majority of seasons so we are confident that this existing squad is more than capable of making the step up."
He added: "With an average attendance of 251 last season, we have a huge following. It's a club that deserves to be playing at a higher level and this is great news for all the players, staff, volunteers, supporters and the town as a whole."
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